I've just reinstalled Oblivion because I decided to try Oblivion XP.

So, I downloaded OBSE, and it's working fine.
I downloaded the newest version of Oblivion XP (OBMM Version), installed and activated it.

But when I'm launching Oblivion from the OBSE Oblivion launcher, half of the bethesda logo is shown, then: "Oblivion has stopped working"

I have even tried launching it from the original Oblivion launcher.

What am I doing wrong?

Did you patch Oblivion to the latest version? (1.2...)
Does disabling Oblivion XP and running Oblivion make it work again? Can't see how this mod could have anything to do with it.

Have a look at the new http://www.gamesas.com/index.php?/topic/1126968-relz-oblivion-xp-update/. I've released a new version. Someone else was having the same problem and some fixes were suggested.