What is the difference between FCOM and OOO? I know that OOO is part of FCOM, but what I'm trying to figure out is if I just want OOO or if I want to go all out and get FCOM.

OOO forms the base of FCOM and basically controls all levelling, skill, and world placement changes. FCOM then integrates all the equipment, creatures and some other stuff from Frans, MMM, WarCry, Bob's Armory, and some unique items from FCOM itself.

Best to start with OOO, then add MMM and then go FCOM later if you find your game is still lacking.

http://www.gamesas.com/index.php?/topic/1094510-relz-fcom-convergence-and-ufcom-relz/

Also worth checking out the latest threads on OOO and MMM in particular, plus anything else you might be adding in along the way.

Essentially, FCOM is (somewhat) an "OOO++" of sorts. A lot more complex to install, especially for those fairly new to mods, so OOO is often recommended as a good way to start out.