It's a combination of things. I've seen video of him, yes. I also know a good bit about basketball in general, OU basketball, OU's team next year, Big 12 basketball, and college basketball. I'm sorry, but as athletic as Osby is, he is only athletic for a 6'8 guy. He isn't a 6'8 guy, with the athleticism to switch off, and guard pg's and sg's (which a true three has to be able to do). Seriously, somebody else asked this question a while back, and there really weren't any good answers. Name a college player that compares to Osby (size and skill-wise), that played mostly at the 3 spot. I'm guessing you can't, or can only come up with a couple. Guys like him play the 4 in college.
And besides, even if coach wanted him to play the 3, a lineup of Cam at the 2 and Osby at the 3 would be terrible. That just isn't going to be a good matchup against anybody. So by default, if both Cam and Osby are going to start, Osby is going to be at the 4.
Watch. That is exactly how it will play out.