Welcome to OBO, the Overall Balance Overhaul, pronounced "oboe" (yes, like the instrument).

OBO is, as the name implies, a balance overhaul the size of which Serious Sam has never seen. Listed below are all the submods that constitute the larger overhaul, allowing you to mix and match as you please. All mods are intended to be used together, and this is recommended, but not required. Forget everything you know about Serious Sam 4, because there's hardly an enemy, weapon, projectile or item that isn't altered in some way (for the better!) by these mods.

Have you ever felt like SS4's balance was "off" somehow? Certain enemies frustrating to the point of infuriating? Unbalanced duel wielding and headshotting making tough enemies not so tough? Tired of thinking "What the heck is hurting me?!" in all the chaos, or suddenly being greeted with a red death screed because you just so happened to not see a single Red Mech rocket coming?

OBO's purpose is to remedy this, to bring back the fairness within the challenge, and with it, the fun.

How does it accomplish this? Well, each individual submod will explain in more detail, but here's the gist of it... No longer will any single piece of weaponry in Sam's arsenal be left to gather dust in his back pocket, obsolete in the later stages! If it's in your hands, it has a place, a purpose, and a niche (even the pistol!). Some weapons were tuned to fill their "intended" roles better, while others (like the Devastator) saw more significant changes.
Duel wielding always granted substantial boosts to DPS (two of the same gun) or flexibility (different gun in each hand), but never had much of a downside, barring exceptions like the Minigun. Now most weapons have some kind of advantage to being held singularly, to try to alleviate the huge impact duel wielding has on balance, while accentuating its strengths.
And as for the enemies, some of the most frustrating elements have been made less so, like Reptiloid green fireballs that never leave you alone, or hitscan snipers. SS4 has several enemies that demand your attention, like Khnums, snipers, Arachnoids, or Processed swarms, and that can be a problem in some of the more chaotic scenes where many such enemies all demand your attention simultaneously. This has been addressed, and staying alive in a frenzy is much more doable provided you're on your A-game, as opposed to the vanilla game where being overwhelmed is common, and hardly feels fair.

Please keep in mind that while you can play with OBO on any difficulty, it is balanced for Serious, and that is why some of the damage numbers enemies deal may seem a bit small. On Serious, all damage received by the player is multiplied by 2, and 1.5 on Hard. Again, this doesn't mean you have to play on Serious, as I encourage anyone to play on the difficulty that works best for you. I just enjoy the chaos of Serious, but always hated the unfairly punishing damage values (such as 300 damage from taking a single direct Red Mech rocket).