Oscar fought May in 2007, before he fought and lost to Mosley in 03, Sturm in 04(he won but really lost if it were judged properly), BHop in 04. He did kill Mayorga in 06 but seriously....who didn't see that coming, Mayorga was an easy win for anyone who was a real boxer (very similar to Naseem Hamed). Then he loses to May in 07 (total of 2 fights in 2 years), followed up with a win over Forbes then to be annihilated by Pacman. The writing was on the wall with his heart not being all the way in it as well as his skills were diminished.
I'm not saying Floyd isn't one of the GOAT but once he reached about 30 wins he carefully hand picked opponents (I personally believe due to Ellerbe) that would be easy wins much like Roy Jones Jr.