I started a new character with oscuros oblivion overhaul (v3) and it seems to be a nice mod, although quite difficult for low level chars.
Are there any other huge overhauls that changes as much as OOO does and wich one is the best experience?

There is Martigens monster mod, Franseco's leveled monsters/items or something like that, Oscuro's oblivion overhaul, Oblivion Warcy, and then there's FCOM convergence that pack all 4 of those together. Here's a video of it http://www.youtube.com/watch?v=Pd6G9zK4D5A. i'm looking forward to this one http://www.youtube.com/watch?v=CoQAQuaEFCg.

What exactly does fcom stand for, I want to download it but can't seem to find it.

Here's a good one for you, "Reneer's Guard Overhaul". It isn't as huge (size-wise) as MMM or OOO, but it does a ton of good stuff.

If you commit a crime in Anvil for example, then only the Anvil guards will know about it (instead of every blasted guard in Cyrodiil).

You can change the fines, and how long the guards remember your crime.

You get a spell to move corpses so the guards won't find them.

Download at: Reneer's Guard Overhaul

Edit - as far as "What's the best", honestly that's a tough one to answer, because if two overhauls happen to be too close to one another in what they modify, then you could very well have a mod conflict. Then the only way to compare them would be to deactivate one then run the other.

MMM & OOO have always been perennial favorites. I don't use either one, as I'm running so many of the huge quest-mods, I don't want anything to have to cause me to remove any of the mods I use. Not saying that it would, just not going to take the chance. But I do use Reneer's mod.

I already use reneers and it is excellent. I found another one, crowded roads, wich is great but is also let down by the fact that there are now too many people using the road, wich makes my PC lag and also I get too much gold at once, you kill one traveller and he has like 50 emarald rings on his body. I would have given it 10 if it just had less people travelling and less loot.

I passed on Crowded Roads. Not because I didn't like it, but I wanted to keep encounters in their pristine locations.

Like for example, when you're walking to a location for the Clavicus Vile Daedric quest. You pass by this seemingly innocent-looking Ayleid Ruin, but if you stray to close to one side of the road.....Wham! It's clobberin' time!