When playing as the Federation at least, there's a Recruitment Academy building that you can build on any planet and that will produce new officers. There are also some edicts that are eventually unlocked that can be activated to increase production. I think some traditions may increase it as well. But Recruitment Academies are the quickest, easiest. I play almost exclusively Federation, but I'd assume most other species work similarly, save for outliers like the Borg.

ok great! Thank you Brian

In addition to what's already been said, starbases in systems with inhabited planets can build a 'Holographic Training Center,' through an early-level tech. Further: on those inhabited planets with a Recruit Academy, adding Education Districts increases the officer production rate by 7%.