Ultra Luminous X-ray Sources (ULXs) are extragalactic X-ray point sources with L_X ˜ 10^39-10^41 erg s^-1 discovered in the 80s with the Einstein satellite and confirmed as black hole X-ray binaries during the last decade. The nature of the compact object is highly controversial. They could be super-Eddington stellar-mass black holes or intermediate mass black holes. Deriving dynamical masses of the brightest ULXs, which can be done with OSIRIS, is the only way to find out the nature of the compact object.