Basics
At this very moment there is 9 classes in total: Officer, Radioman, Rifleman, Assault, Support, Flamethrower, Engineer, Machinegunner and Sniper as shown in the screenshot below:
Their names pretty much tell what they are for, like for example Assault assaults the enemy lines, Support gives their allies suppressive (support) fire and engineer works with explosives to blow sh*t up. To make them diffrent and unique from each other, they get diffrent weapons, trowables and even perks. More about them later in the guide...

Officer is the leader of the team, his abilities can change the direction of the match. Be the inspiration for your troops.

Officer is probably on of the most important classes in the whole game because of his ability to call in supply drops, artillery strikes, air support and etc.. Along with that he gets two new UI windows. One of them allows the officer to command his troops (more about this on advanced officer guide) and other allows you to call in radio support:

Along with that, Officer class get's cometic and diffrent head wear. British officer gets beret, Wehrmacht officer gets a officer hat and U.S. Army officer gets a white stripe on the back of his helmet.

Officers perk as mentioned before allows him to call radio support with a help with another player with a radio as seen below:

Along with that, there's a small radio tower icon on the player with a radio to help the officer to find him faster. It shows small white radio tower, the player's name and how many meters away is he:

The supports he can call in are:

• Explosive Barrage
  
• Smoke Barrage
  
• Carpet Bombing
  
• Supply Drop
  
• Special Support (British get Typhoon Run, U.S. get Gun Run and Germans get Stuka Dive Bomb)
  
• Incendiary Barrage (only commonwealth army gets it, but it's counted as a other two barrages)

Every of the supports have diffrent stats and certain of them share cooldowns. Basicly if you use, let's say, a Smoke barrage, then explosive barrage will get a cooldown (incendiary barrage will have cooldown too if you're in commonwealth faction). After calling each support, you won't be able to call in support again for about 15 seconds, that's the global cooldown. It is not shared between the officers.

U.S. Army
Primary: M1 Garand, M1 Carbine, M1A1 Thompson (M1A1 Carbine with paratroopers)
Secondary: Model 1917, M1911
Explosive 1: Smoke, Mk.II
Explosive 2: Smoke, Mk.II
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Commonwealth Army
Primary: Lee-Enfield No.4, Sten Mk.V, Thompson M1928A1, Owen Gun (with ozzies)
Secondary: Welrod, Webley, Hi-Power
Explosive 1: No.79 Smoke, No.36 Mills Bomb
Explosive 2: No.79 Smoke, No.36 Mills Bomb
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Wehrmacht
Primary: K98, Mp-40, G43
Secondary: P-08, C96
Explosive 1: Nebelhandgranate, Steilhandgranate, Splitteringgranate
Explosive 2: Nebelhandgranate, Steilhandgranate, Splitteringgranate
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

• Try not to get exposed too much when you're calling an arty. If you manage to call something in, but die before your character finishes saying his request, it will be canceled, since the station "couldn't understand your request".
  
• Communicate with your team. Always having a radio next to you allows you to communicate with most of the team, since there's 80 % chance that there will be always a player with radio attacking the point along with others. Tell them what to do, point out campers and their positions. Knowing the game well will help greatly.
  
• Know your maps. Knowing where to call that fatal Carpet Bombing that will kill almost a whole wave of enemies is one of the most rewarding and satisfying things in this whole game. Know your maps, think where the enemies could be.
  
• If there's enemies capturing your objective, never call in an arty on it. It might be a good idea if you can communicate with your team and tell that to stop, but most of the time some players will blindly run into the objective without knowing that there's firesupport coming. Call in special support instead! It arrives and gets the job done faster, which is very important when every second of enemies being in the objective counts.
  
• Try calling in support with iron sights more often. It's a lot faster and safer than using the binoculars, because you need to peak out for good few seconds to call in fire support. Simply aiming your sights, pressing the Radial Messages 2 times and then pressing on the support you want is a lot faster. But binos shouldn't be put on the shelf either, since still you can search for hidden snipers or MG's and call arty on them with better accuracy in long ranges.
  
• If you accidently missed a explosive artillery or a bombing run and you know it, just quickly call in either a supply drop near you or a support that still has cooldown on your position BEFORE you character ends his request to the station. That will hurt noone and you will be able to call in that support in the right place you wanted in the first place (after waiting global cooldown).
  
• Gun Run's, Carpet Bombing's and Typhoon Run's area starts from the point where you called them in and goes forward. Basicly what this means is that if you called a carpet bombing directly on a house, the bombers will drop few bombs on the house and the rest bombs will go behind it. It's extremely useful knowing this, since you can cut off enemy reinforcements moving into a point, simply by calling few bombers directly on it.
  
• Explosive, Incendiary, Smoke Barrages, Stuka Dive Bomb and Supply drop will go directly at the place you pointed. The barrages drop a certain area around the place where you originaly called them in.
  
• If you're attacking a objective that is in open field (like Sicily's A, Bastonge's A or Saint Lo's A) it's best to call in smoke (or Incendiary if you're british) BEHIND the objective (in your spawn persepctive). This way long range MG's, engineers with rocket launchers or snipers won't be able to kill you and your allies from a distance, plus you can see a attacking enemy in the smoke a lot better than they can you.

PS: I will make a guide fully about radio support, but later (never :P).

Radioman is the bodyguard of the officer. Do everything it takes to protect him and to help your team.

Radioman is best officers friend. With his help, officer can call in artillery, ammo and ect.. Along with that he can capture points faster and talk with another players who have a portable radio on their backs. It's highly recommended for the radioman to stick with his officer, so he can help his team with smoke barrages and ect.. Also he is very useful as for assault as his radio gives him the ability to capture points faster, but more about that on Assault section of the guide.
Officers for the radioman is shown as white three rank strippes icon as shown below:

Also the icon show's his name and how many meters he is away. It's very similar to how Officer sees Radioman.

His perk is basicly a free radio. Along with that he gets cheaper smoke grenades and cheaper sidearms.

U.S. Army
Primary: M1 Garand, M1 Carbine (M3 Grease Gun in co-op and M1A1 Carbine with paratroopers)
Secondary: M1911 (Model 1917 in co-op)
Explosive 1: Smoke, Mk.II
Explosive 2: Smoke, Mk.II
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

Commonwealth Army
Primary: Lee-Enfield No.4, Ithaca (Sten Mk.V in co-op)
Secondary: Webley, Hi-Power
Explosive 1: No.79 Smoke, No.77 WP Gernade, No.36 Mills Bomb
Explosive 2: No.79 Smoke, No.77 WP Gernade, No.36 Mills Bomb
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

Wehrmacht
Primary: K98, G43 (Mp-40 in co-op)
Secondary: P-38, PPK
Explosive 1: Nebelhandgranate, Steilhandgranate
Explosive 2: Nebelhandgranate, Steilhandgranate
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

• Try to cover your officer when he's trying to call in something. The this is that if he starts calling in support and he dies, the support will be canceled. But if you, the radioman, die and the officer lives, the support will arrive althou the radio is dead. Keep this in mind while playing.
  
• While radio is great at capping points and all, it's better to pick a Assault class or a Rifleman class with a radio on their backs, Because they are just better at combat in general. More about that on Rifleman's guide and Assault's guide
  
• Work with your officer. Cheap smokes for officer, you and the ability to call in a smoke barrage is an extreme push force which can cap any objective if even two guys are coordinated.
  
• "Building" your loadout isn't that much of a hard task to radioman, but you always need to have one thing in mind: Always pick grenades and sling on our primary weapon over everything. You're one slow motherducker and having a sidearm (which is cheap) is great and all, but extra smokes (and especially WP Grenades for british) is a very great way of protecting your officer when you're defending or just pushing up and cleaning your objective in Offensive / Invasion.

Rifleman is the blood and bones of the team, push the line using your accurate rifle and deadly grenades.

Rifleman is the the base of every game, the simple, but if played right, effective class. He is made for medium range battles and is one of the main pushing forces in the game. Althou many would say that SMG's are the base of this game, really it's the rifles that make the game interesting. Rifleman takes long time to master since most of the time he is running around with a bolt-action rifle, hard to use, but rewarding gun (U.S. might get a semi-auto M1 Garand, but that gun needs skill too, since it's fair share of recoil and the fact that you need manage you ammo pretty often is what makes that gun sometimes equal with bolt-actions). This class just encourages using rifles over everything else. It's a great choice if you want to learn how to play with bolt-action rifles.
In addition to that, he is the only class that gets rifle grenades: smaller, but further range grenades that are shot from your rifle in a cost of long reloads (trowing grenades is faster, but rifle grenades travel further) and the inabillity to bash (this is more of a miss on NWI side, since it doesn't make much sence that you can bash if you have picked rifle grenades. I guess certain things in the engine doesn't allow that...). Also he gets cheap, 1 point upgrades to his rifles (like wank tube and bayonet which cost 2 supply points for all other classes), but to balance that out, his sidearm cost the most out of all other classes.

Rifleman perk makes him one accurate son of a bench. Basicly what it does is make his rifle sway a lot less and hold his breath the same amount of time when he's tired from running, BUT he will still get suppression effects from enemy fire. But he is still very good, having the ability to shoot straight after running a few hundred meters is extremely useful.

U.S. Army
Primary: Springfield 1903, M1 Garand
Secondary: Model 1917, M1911
Explosive 1: Smoke, Mk.II
Explosive 2: Smoke, Mk.II
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

Commonwealth Army
Primary: Lee-Enfield No.4
Secondary: Webley, Hi-Power
Explosive 1: No.79 Smoke, No.36 Mills Bomb
Explosive 2: No.79 Smoke, No.36 Mills Bomb
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

Wehrmacht
Primary: K98, (G43 in co-op)
Secondary: P-38
Explosive 1: Nebelhandgranate, Steilhandgranate, Splitterringgranate
Explosive 2: Nebelhandgranate, Steilhandgranate, Splitterringgranate
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

• If you want to pick radio on rifleman class to help the officer, I suggest you only doing this if you're have good amount of points and you're just good with bolt-action rifles. Althou, riflemans steady aim perk does come in handy, it's better to have few more grenades or smokes (because they are cheaper and radio doesn't cost anything) as I mentioned in Radioman section of the guide. But if you're want to take the radio just for the sake of attacking and capturing the objectives faster, do it, rifleman is a bit better than radioman in the terms of combat (even if radioman get's semi-auto rifles like Wehrmacht or Commonwealth do).
  
• Aim for center of the body. One shot from bolt-action rifle (and M1 Garand) is fatal even to the whole chest, so there's no point in trying to land a headshot like some kind of showoff. It's better to shoot someone quick than make a beautiful kill.
  
• Learn the art of flicking. Video footage showing what is a flick:
  Basicly you just quicly turn your gun torwards your enemy and shoot without aiming. It's a good tactic / skill to use in CQC combat with bolt-action rifle. Knowing how t and being able to flick allows you to quickly kill you enemy if you're getting attacked from a side in tight room / house.
  
• The best early game loadout for the rifleman is getting upgrades such as stripper clips, sling and bayonets and one grenade (U.S. default loadout is good already). Althou it depends what is better: having additional grenade or having better rifle. I personally prefer having more grenades if I am attacking and having better rifle if I am defending. But it's just a personal prefrence thou.
  
• Learn how to use bashing. Bashing is a great mechanic in DOI. It allows you to get away from situations when the enemy is near you and you are out of ammo or you shot him and he survived. This is especially useful for rifleman who needs to cycle his bolt after every shot he does (or to reload your garand when you're dry).
  
• Learn how to use bayonets. Using bayonets is pretty tricky, since you need to know your maps to know where is smart to use a bayo and where is not. Friendly advise: Never spam bayonet to stab people, unless they are injured and you know it! It's always better to calmly charge up your bayonet and stab your opponent. This way you will not only will have higher chance of hitting him (because of charged bayonet mechanics), but also if you will hit him, he will be dead for good.
  
• For late game I advise picking more explosive grenades over smokes or a sidearm. Remember that your long range accuracy is a advantage over those pesky SMG's, thus making no need for smokes, since you can shoot long rnage enemies yourself. And overhaul, grenades are just generally better for getting campers out of buildings and cleaning out cap points, since that will be the most often thing you will be doing as a rifleman.

Assault is build to flank enemy positions using his smokes, fast speed and fast firing guns.

Gotta go fast! Assault's role in DOI is to flank and clean areas effectively in close range. His main weapons are SMG's which exchange damage and accuracy for ROF and light weight (exept for M3 Grease Gun, that gun is special, but more about that in Weapons And Equipment Guide).
Along with that, Wehrmacht and Commonwealth Army light vest for assault costs only 1 supply point, compared to other classes. This allows him to pick good SMG and a grenade from the start of the game. Oh and the U.S.? They get the middle finger (jk, I guess it's for balance).

Assault's perk gives the player higher speed than any other class. He can faster than others even with a lot of heavy gear, which makes him very good at quickly attacking enemy positions.

U.S. Army
Primary: M3 Grease Gun, M1A1 Thompson
Secondary: Model 1917, M1911
Explosive 1: Smoke, Mk.II
Explosive 2: Smoke, Mk.II
Utility:
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

Commonwealth Army
Primary: Ithaca, Sten Mk.II, Owen Gun (with ozzies) (Sten Mk.II-S and Thompson M1928A1 in co-op)
Secondary: Webley, Hi-Power
Explosive 1: No.79 Smoke, No.69 Grenade
Explosive 2: No.79 Smoke, No.69 Grenade
Utility:
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

Wehrmacht
Primary: Mp-40, C96 Carbine
Secondary: P-38
Explosive 1: Nebelhandgranate, Steilhandgranate, Splitterringgranate
Explosive 2: Nebelhandgranate, Steilhandgranate, Splitterringgranate
Utility:
Vest: Light Kit, Heavy Kit
Backpack: Radio Kit
Melee: Knife

• Pick and use smokes. Because of SMG's limited accuracy and range, most of the time you can't kill peeking enemies especially if they are suppressing you. Use your smokes to blind, flank and fill their *sses with lead.
  
• Radio + Assault = Ultimate objective capturing machine! While radio does slow down assault a good amount, he is still cappable to run faster than let's say a rifleman or a radioman classes. This makes him quick at everything: shooting, running and capturing.
  
• The Bezerk (I gave him this name, deal with it). The bezerk is a tactic in Insurgency used by good competitive players. It's pretty simple: Don't pick ANYTHING apart from your weapon. This makes you extremly fast. Along with assault's fast movement perk, you're literally Sonic The Hedgehog, running around like it's the 90's! This will allow you to move exeptionaly fast on enemies positions and flush them out. Recommended weapons for this tactic are: MP-40, C96 Carbine and M1A1 Thompson (Thompson M1928A1 in co-op). Everything that shoots extremly fast basicly. Sorry Sten and McGrease :(
  
• Aim for the head. Because of the high ROF of some SMG's, it's better to aim for the enemies head if you're close enough and he has a one shot rifle ( a gun who can kill u in 1 shot). This way, you weaken his chance of shooting you back. The most ideal weapon for this is Mp-40, because of it's balance between accuracy, recoil and ROF. M1A1 Thompson just likes going off target way too often...
  
• Always look for upcoming cooked enemies grenades. Because of your fast movement, you can go pretty quickly out of place and trow the grenade somewhere away. And even if the grenade explodes shortly after being trown back, it won't damage your allies as much, because it will be addressed as your grenade (friendly fire makes less damage to your allies than enemy fire)

Support is the medium / long range automatic rifleman. Use his powerful weapons and lot's of smokes to help your team advance.

The automatic sniper of DOI. The mini MG. The "whatever name you can think for it". Support's role is to suppress / kill enemies from medium range. He is something between rifleman and assault with medium range automatic LMG / assault rifles. Along with that, he gets cheap, 1 point smokes and cheaper Heavy Kit (4 supply points instead of 5)

Althou it says that support per kis that he gets extra ammo, it has been confirmed by NWI that support doesn't get any perk at all (this can be proven by the facts that he still get's 4 pistol mags and 5 primary weapon mags with light kit like everyone else).

U.S. Army
Primary: M1918 BAR
Secondary: M1911
Explosive 1: Smoke, Mk.II
Explosive 2: Smoke, Mk.II
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Commonwealth Army
Primary: Bren Gun, Lewis Gun
Secondary: Webley, Hi-Power
Explosive 1: No.79 Smoke, No.36 Mills Bomb
Explosive 2: No.79 Smoke, No.36 Mills Bomb
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Wehrmacht
Primary: Mg-34 (FG-42 with Fallschirmjäger and Stg-44 with 17th or 15th panzer division)
Secondary: P-38, PPK
Explosive 1: Nebelhandgranate, Splitterringgranate
Explosive 2: Nebelhandgranate, Splitterringgranate
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

• Use your cheap smokes. It's always important to cover choke points so you can pass on certain areas.
  
• Pick Sling on your gun. Default Commonwealth gives you 2+1 smokes with a heavy kit and a bren gun. It's a lot better to pick Bren with sling, 1 smoke and plus the smoke that you get from heavy kit. Same goes for U.S. Army support's loadout, but for him, it's better to get rid of heavy kit, pick 2 smokes and a BAR with a sling.
  
• Use full-auto on FG-42. That gun is actually pretty inaccurate with it's semi-auto mode, so it's a lot better to use automatic fire. Few would say that it's only my personal prefrence, but it's not, it's just inaccurate on semi-auto and that's all.
  
• Manage your ammo. All support guns (exept for Stg-44) have longer reload times than SMG's or rifles. You need to count your bullets and try not to reload too often, since you will end up with a bunch of mags with few bullets left. It's better to shoot them dry out.
  
• Use semi-auto on MG-34. It has the longest reload in the whole game, but only 50 rounds in it's assault drum. If you use it as MG-42 and just spam the fire, you will spend more time reloading in cover than shooting. Be smart about it.
  
• Use bipods as often as possible. They kill almost all recoil from your weapon and allow you to shoot at long range targets!
  
• Burst fire on the enemy. Never go full auto on enemies or you will be out the in the next 3 seconds. It's better to shoot at least 3-5 bullets at one enemy. Most of the LMG are one hit kill in most of the ares, so that means there's a big chance one of those bullets from burst will kill him. This applies to guns like FG-42, BAR, Bren and Lewis. Stg-44 is completly diffrent story, since it acts more like a SMG's than a LMG like other support weapons.
  
• In late game, focus on more ammo (heavy kit) and smokes. Grenades and sidearm should be only picked as the last things you need, but if you're assaulting the enemy with a support, then it's better to have a mix of everything: a smoke, grenade, a pistol and a primary.
  
  

Easily clear building without giving the enemies a chance to shoot you back. Use your smokes to cover yourself from long / medium range enemy fire.

Did someone mention BBQ? Flamethrower class is made for two purposes: Getting campers out of tight spots and having a lot of fun. Both of these things makes the class unique and fun to play (duh). The flames from his guns can bounce off surfaces, thus giving him the ability to burn enemies without seeing them. But althou having the perfect weapon for building cleaning, this class hits three main problems that makes him not so OP as he looks in the first place. First, the flamethrower doesn't have a lot of range, so everything that is capable to shoot him from a distance wins the battle most of the time. Second, the fuel that the flamethrower uses is extremly heavy, thus making the class very slow. And at last, the flamethrower weapon is as dangerous to you as it is to your enemies. The flames can burn you to death in a matter of seconds, so if you're injured and use the flamethrower incorrectly, you're gone out of the living world.

Flamey's perk is that he is the only class that get's flamethrower. Along with that he get's very heavy and inremovable fuel tank which is the flamethrower's "ammo".

U.S. Army
Primary: M2 Flamethrower
Secondary: Model 1917, M1911
Explosive 1: Smoke
Explosive 2: Smoke
Utility:
Vest: Light Kit, Heavy Kit
Backpack: Fuel Tank
Melee: Knife

Commonwealth Army
Primary: Flamethrower, Portable, No 2
Secondary: Webley, Hi-Power
Explosive 1: No.79 Smoke
Explosive 2: No.79 Smoke
Utility:
Vest: Light Kit, Heavy Kit
Backpack: Fuel Tank
Melee: Knife

Wehrmacht
Primary: Flammenwerfer 41
Secondary: P-38, C96
Explosive 1: Nebelhandgranate
Explosive 2: Nebelhandgranate
Utility:
Vest: Light Kit, Heavy Kit
Backpack: Fuel Tank
Melee: Knife

• Althou at first it seems like Flamethrower class is very easy to use. But that is not the case. This class needs you knowledge of the maps and their camping spots in certain areas, so you can pre-fire your enemies with your bouncy flames. This is the best way to play the flamey class, because the flamethrowers themselves act a bit weird in DOI. Basicly the flames reach your opponent after a second when you pull (or push) the trigger althou it looks like they reach him right away. And if you pre-fire your enemies, there's a very small chance that they will have a chance to shoot you back.
  
• Enemies can survive after being lit up by flamethrowers. Because screw the logic and hail the balance! To make this a rare occurance, team up with your officer and the rest of the team, so they use artillery, grenades and whatever that can injure the enemy. A injured foe has no chance of surviving fire that ignites him, even if he kills you after you lit him up.
  
• Use smokes to cover yourself. As I mentioned before, Flame boy is very vulnerable from small distance, so it's not that hard to shoot him without giving him a chance to threaten your life. Use smokes to block of the view between you and the enemy.
  
• Be careful around corners. Since flames bounce off all surfaces, you can accidently burn yourself. This especially applies to situations, where you're shooting while standing next to the corner or shoothing through a windown. If your end of the hose ends up too close a wall / side of a wall, the flame will bounce off and slowly kill you.
  
• Watch's those friendlies. If you shoot your flamethrower and a teammate runs in your way, you're dead. If the impact of the flame you shoot is near you, it will give you lot's of damage ending in a toasty.
  
• Extra Tip! If you want to become a extreme alchoholic, take a shot of vodka everytime I mention the word "Flamethrower" in this guide.

Engineer's solely purpose is to use his rocket launchers and explosive charges to destroy walls (the destroyable ones duh), objectives or just clean certain spots from campers with good aim and a push of a button (by shooting a rocket obviously).

Just because there's no Micael Bay in WW2, it doesn't mean there's will be no explosions! Engineer is basicly a simple soldier, armed either with a bolt-action rifle or a SMG along with his valuable explosives. Now he can choose from two main types of explosives: TnT Charge which is powerful, but can only be trown few meters away or a rocket launcher, which is capable to fire a explosive rocket into any direction from any distance (exept for PIAT, it has huge drop off, which hit's it's range very hardly, but to compensate for that, it never goes even a centimeter to the side and get's a third bomb with heavy kit)

Engineers perk allows him to carry more weight onto his body. He can carry multiple mags to his primary and secondary weapon, few rockets to his launcher and a additional grenade. Also he gets cheap grenades and sidearms.

U.S. Army
Primary: Springfield 1903, M3 Grease Gun
Secondary: Model 1917, M1911
Explosive 1: Bazooka, TnT Charge, Mk.II
Explosive 2: Mk.II
Utility:
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Commonwealth Army
Primary: Lee-Enfield No.4, Sten Mk.II
Secondary: Webley, Hi-Power
Explosive 1: PIAT, TnT Charge, No.69 Grenade, No.77 WP Grenade
Explosive 2: No.69 Grenade, No.77 WP Grenade
Utility:
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Wehrmacht
Primary: K98, Mp-40
Secondary: P-38, PPK
Explosive 1: Panzerschreck, Sprengbuchse, Panzerfaust, Einstoss-46, Splitterringgranate
Explosive 2: Einstoss-46, Splitterringgranate
Utility:
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

• All launchers (exept for PIAT, as mentioned before, and Panzerfaust, because it has drop off and smaller explosion) are very inaccurate and most of the time fired rockets tend to go a bit up, down and to either of the sides instead of the straight destination. To decrease chance of a miss (if you're shooting a rocket into thight window) try coming as near to it as possible and before shooting aim at the very center of the window. This way you give your rocket the best chance to reach it's target without a miss.
  
• Think fast. All rocket propelled explosives (and PIAT) have a slight delay between the trigger pull and the explosive being shot. Basicly if you pull the trigger and get killed soon after, the shot won't count as the delay wasn't over. To prevent this from happening, shoot as soon as possible!
  
• Plant your explosives! There's two ways of activating a explosive charge: Trowing it with Mouse1 or planting it with Mouse2 button. Planting is the best option if you have some time and you reach your target that you want to explode. This way the enemies can't trow the TnT back.
  
• If you're really desperate for TnT to explode as soon as possible, shoot it with your gun. Shooting it will detonate it instanly and get the job done without any waiting.
  
• Default starting loadouts for Engineer are good if you're can and want to play with bolt-action rifles. But if you need that expensive SMG as soon as possible, just get rid of your light kit! Just pick a SMG and a rocket launcher and you're good to go. Not only you will still have your 2 rockets, but also you will be a bit lighter and you will have a SMG as engineer from the very starts of the game (with a con of having 3 mags on your primary weapon and no Explosive 2).
  
• This tip applies to co-op only! If you're a german engineer and need to destroy a fuel dump / generator very quickly, it's very easy to do by firing your Einstoss-46 to it! This tip has been advised by another experienced DOI player Fergus.

Machinegunner is good shooting his gun for long periods of time and from any range. But know your bullet count, as those ammo belt's ain't infinite! Good pistol skills advised.

The name "machinegunner" pretty much says the role of this class: It has a GPMG which is capable of firing for long periods off time and from any range, if it's setted up correctly. This class requares somewhat map knowledge (at this point, everything does) so you know where to set up your MG as being too exposed or in a wrong / bad spot will cost you fun and efficiency. Also you need to have good sidearm skills, because the inability to ADS without putting the gun on bipod and high recoil makes the MG almost imposible to use on a move. Pistol is the way to go.

MG's perk gives him resistance against suppression from enemy fire. Basicly his aim, accuracy and sway isn't effected that much from suppression effect (althou being tired still affects you). Along with that he gets cheaper Light Kit (1 supply point) and cheaper Heavy Kit (3 supply points), so he can carry more ammo for his sidearm (the MG belt amount isn't affected).

U.S. Army
Primary: M1919 Browning
Secondary: M1911
Explosive 1: Smoke, Mk.II
Explosive 2: Smoke, Mk.II
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Commonwealth Army
Primary: Vickers
Secondary: Webley, Hi-Power
Explosive 1: No.79 Smoke, No.36 Mills Bomb
Explosive 2: No.79 Smoke, No.36 Mills Bomb
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Wehrmacht
Primary: Mg-42
Secondary: P-38, P-08
Explosive 1: Nebelhandgranate, Stielhandgranate
Explosive 2: Nebelhandgranate, Stielhandgranate
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

• As mentioned before, know your map and good places to set up. Althou I could show one of the best spots to sit with MG, you really need to figurate such things yourself by playing the game.
  
• Going rambo in tight spaces like Dog Red Bunker or one of the Crete houses, but I would suggest you to stay with your pistol as often as possible when you're on the move. Just before going into place you want to deploy, get your primary ready.
  
• Crawling with you bipod setted up is actually a pretty decent tactic on open, but hilly maps (like Bastonge, Sicily or Foy on some places). You will be moving always ready with your gun to shoot back, but ofc with huge movement penalty.
  
• Pick a M1911 instead of a smoke! Default early US MG loadout gives you your MG, a smoke, light vest and a spare point. Instead of that, pick a pistol and a primary, there's no point in picking that smoke if you don't have a sidearm to defend yourself on the most situations.
  
• Fire in bursts. Burst, as mentioned before,

Sniper is pretty much a long range rifleman. His role is to kill MG's, defending supports, other snipers and whatever is on his scope. High skill level with a sidearm needed.

No quickscopes here kiddo. Sniper is the horror of the defending team. His long range scope obviously gives him the ability to see and shoot enemies from very long ranges. On maps like Dog Red, Foy or the start of Bastonge. Althou having a powerful scope, he lacks good primary weapon for CQC and short range combat (using a scope is way too risky, if you don't have 4x scope). Then his handgun comes in handy!

Sniper perk allows him to hold his breath for longer periods of time than other classes (holding breath (SHIFT by default) makes your weapon more accurate and sway less for certain amounts of time). This allows him to land accurate shots on his targets. But enemy suppression effects still apply to him and still decreases his focus time (but it's still longer than other classes while suppressed).

U.S. Army
Primary: Springfield 1903 (M1 Garand in co-op)
Secondary: Model 1917, M1911
Explosive 1: Smoke
Explosive 2: Smoke
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Commonwealth Army
Primary: Lee-Enfield No.4
Secondary: Welrod, Hi-Power
Explosive 1: No.79 Smoke
Explosive 2: No.79 Smoke
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

Wehrmacht
Primary: K98, G43
Secondary: PPK
Explosive 1: Nebelhandgranate
Explosive 2: Nebelhandgranate
Utility: Binoculars
Vest: Light Kit, Heavy Kit
Backpack:
Melee: Knife

• The long sniper scope isn't always that great, since some maps are just too tight around places or too short in ranges (like Saint Lo, Comacchio and the german side of Sicily and Foy). Then the 4x scope comes in handy. Althou it doesn't have such power full zoom as the 7x Scope, it's a lot better to use it for medium ranges and still be as useful for your team as with the powerful zoom.
  
• Pick Hi-Power instead of the Welrod. While welrod is sometimes great, it's accuracy is pretty useless, when you have a sniper rifle. It's a lot smarter to have semi-auto pistol like Hi-Power for CQC situations.
  
• Default early sniper kit among all factions always gives you a rifle and a 7x scope with spare supply point. Always remember to spend that point on a sling!
  
• Never use sniper without a scope. It's a lot better to pick rifleman, if you want to play with normal bolt-action. He has cheaper upgrades and his perk is better for bolt-actions overhaul.
  
• Always be for a lookout, never look at one point for too long. Keeping your eye on spots where enemies could be is always a great idea.
  
• Don't stay in a one place, advance with you team. Camping sniper that sits in one place is never a good thing. Move with your team and try to help them in everyway possible.

Long time ago I said I'm going to make a bunch of guides that will eventually cover the whole game..... Now I can say that that won't happen for few reasons I'm gonna mention here:

1. The lack of feedback and views. Literally a stupid guide which was about making bloody pasta was posted almost the same day as this guide and it has x10 the views and favorites. People don't get into this guide as much I wished, but there's logical explanation for that which is....

2.Everyone has their opinion and way of playing. This guide is very subjective what you need to do and what you don't need to.... which is bad, since diffrent people feel differently about the same things, like using, let's say, a class in a way others would think is completly dumb. It's basicly arguing about a fact which makes both of you right.

3. The amount of grammar mistakes I make. Besides typos (which I make too many) writing words and making sentence structure from my native language to english is pain in the *ss. I'm not the best at english itself either, but the point is: Who wants to sit few hours reading and actually following something as grammaticly broken as this? I might be exaggerating, but it's still a big factor why these guides ain't as good as they could be.

4. The fact, that you need to learn DOI yourself and feel it. It's just such game, you won't become a pro (like me ( ͡° ͜ʖ ͡°)) after reading few pages of tips how to git gud, you need to feel how guns handle, how enemies act in certain maps and etc.. And the last important fact:

5. The amount of guides and videos telling you what to do. There's lots. Like lot's lot's. All of them are pretty similar and only few bring something new to the table..... but all of them are seen as the same thing most of the time, which makes most of people go "meh" when seeing such guide.

If you got dissapointed, well sorry, I don't see the point in doing those, although I still have few projects in mind which are less ambicious (and boring :P), but more entertaining and interesting..... Coming soon...... Sooner than MAX GUIDES!™ part 2 hopefully :D...