I agree with all the NCO suggestions above.
For junior officers = Appropriate template (ie Marine, Army, Navy, Air Force) + MOS Training + Officer Training
For senior officers (perhaps Colonel and above) = Appropriate template + MOS Training + Officer Training + War College
Because senior military officers were junior officers at some point, they would require both Officer Training and War College, not instead of.
The US Army War College at Fort Leavenworth is a school for Colonels and General Officers (for example).
Another consideration for officers, that while mentioned above officers generally must possess a Bachelor's Degree in order to qualify, there also exists military universities for US like West Point, Virginia Military Institute, the Air Force Academy, and Anapolis Navy College (which would apply to both navy and marines).
While there also exists Officer Candidate Schools from elevation of enlisted to officers, not every military, nor perhaps your game system having that available.
USA War College was required for my dad to make major. Hence why I know about the correspondence course. (He was a reservist.)
And the US has 4 methods of officer creation:
Academy: attend as college, but includes many service related skills, and counts as initial entry training
Officer Candidate School: start as enlisted, go to OCS.
ROTC: attend a college, and get the same skills as an academy grad (but at different levels, most likely), while having a more normal college experience.
Direct Commission: go to college, get a degree in something the military needs, and go direct to a "conversion course" that is equivalent to OCS... in the US, it's mostly chaplains, lawyers, doctors, and nurses. Of these, only nurses aren't typically bachelors + graduate school.
Some countries services historically had direct NCO's as well. The US, for a while (about 2 years) made enlistees to the Army Air Corps Flight Sergeants right off if they already had a pilot's license and could pass the checkride. (Congressional meddling resulted in most of them becoming Flight Officers {Warrant Officer 1} and barring sergeants from flying combat missions.)
The Germans and Russians both had junior college academies that turned out direct NCO's, and their top few percent went to officer academies, at various times. So an Unteroffizier might be a veteran private promoted, or a junior college graduate with under a year in the army... while his classmate who did better went on to an academy and became an officer.
A surprising number of militaries historically had direct commissions to colonel and general. You paid for your rank, so if you could afford it, you could be it. A few still do (but they're typically backwaters).
Also worth mentioning - not a few nations don't have their own militaries, or have only ground forces, and rely upon a patron nation for military presence or naval presence. (EG: Federated States of Micronesia - the FSM has, per capita, more people in US military service than any US State! The US provides their Army and Navy presence, when needed, and their nationals are allowed to join and serve in the US military - but not above O-6 (Army Colonel, Navy Captain).
