I have OOO, MMM, Realistic Leveling, but what other epic overhaul-like mods should I get?

Unless you are installing overhauls that are intended to be used together, do not do it. You will have a huge compatibility mess on your hands. Anyway, the other big overhauls are Tamriel Immersion Experience (TIE), Francesco's New Creatures Items Mod (Frans), Oblivion WarCry and FCOM. When I see the word "epic" I start thinking quest mods though... The only one of the aforementioned mods that is not completely supported by FCOM (yet) is TIE. TIE4MOD (FCOM really) is being worked on, however.

Edit: RL is neither epic, nor is it an overhaul, haha. What are you looking for now?

FCOM would add WarCry and Francesco's to your setup. Outside of FCOM, it's an either/or proposition.
As to all the other overhauls, you'll have to choose whether to switch to them or stick with OOO. You can't just add them together.

gothemasticator

MMM and OOO work together though without FCOM, right?

Neither Frans nor WarCry are http://devnull.sweetdanger.net/fcomloadorder.html and, yes, OOO+MMM is a separate, already supported overhaul combination.

Edit: Helped someone install http://www.gamesas.com/index.php?/topic/1083199-martigens-monster-mod-ooo/ with BAIN, quite recently.

I can recommend going with FCOM - it is awesome.