@Freestone
If your running a rig that's up to standard then the mod won't have a noticeable effect by itself on FPS.
The big resource eater will be the high rez graphic replacers. If your worried about large spawns use one of the reduced spawn options.
I would also strongly suggest if this is the first time you've played Oscuro's then don't run it along with DLL if you want a true OOO experience.
Yes, DLL has great potential, and I wish Leviathan well with it's development, but it would significantly alter how the mod plays.
I second the recommendation not to run DLL if you want the OOO experience. Having recently played it both ways, OOO spawns have a logic to the type of creature spawned and where. DLL on the other hand looks at level list and spawn creatures that are on the list, without considering what type of creature. This alters some of the faction fighting which is part of the non-player centric world OOO creates, where you may find caves with a bunch of dead critters (just watch out for the one that killed THEM). It also allows you to survive by leading creatures/NPCs from one faction to a rival faction, thus helping you in the fight.
If on the other hand you are just looking for creatures from OOO and not worried about the feel of the overhaul itself, then DLL is the mod that will show you everything OOO has to spawn, configurable to allow spawns from all the lists at any level (so you do not have to wait until you are level 30 before seeing a creature from that leveled list).
OOO is definitely worth playing on its own though. If you read the books and notes you find you will find a well thought out world.