OOO is my favourite mod, which changes a LOT of things.
Basically, in respect of the leveling system, it removes the leveling list. This means you can encounter any leveled monster at any level, and some caves are set for low levels and some for high.
This means that at level 1, you can encounter caves (which are usually far off into the wilderness) with level 30 monsters. However at level 30-50, you still encounter bandits wearing fur armour on the road.
So instead of trying to cripple yourself by picking attributes you'll never use to cap your leveling, you focus more on the skills you will use.
It makes the game 10x more challenging, but with the massive amount of content it adds... I couldn't play without it again.