Yes its excellent imo. I wouldn't play the game without it. No more silly level scaling with this mod. It changes quite a bit, leveled lists, loot etc are done by hand by the author in a lot of cases. You don't get ridiculous things like Bandits wearing high end armors, wolves and rats forever scaling with you etc when you gain levels. And you can't beat the Arena at a low level like in Vanilla OB either.
So many things this mod changes that you'd need to read the readme to see ( its a big readme, lol). It works great in conjunction with a level slowdown mod (a few included) because monsters/npcs spawn more and you'd fly through levels otherwise. O3 is also compatible with a lot of other good mods. The only no no is using it with a similiar leveled list mods of course.
Another great thing is some awesome mods like Natural Wildlife (spawns more and varied wildlife) and Combat Behaviour (makes fights more unpredictable etc) comes built in to O3
But you might want to hold off for a week or so as the new version of O3 is due soon and promises a hell of a lot more than the current version. Tons of user made armors etc that have been hand placed into leveled lists and loot containers. And stacks of other awesome tweaks