I think there is a decent chance for either (Collins or Oscar) returning.
Why?
Look at Cockburn this summer, 7'0 AA post-player, and there was not enough NBA interest for him to stay in the draft. So unless Oscar can play outside some, at least defend outside, then I don't see him having more than 2nd round interest by NBA teams (& even 2nd round is far from a guarantee).
Oscar is going to have a tough time getting minutes this year IMO, and may be in the 10-15mpg range, possibly less. That does not mean he can't be drafted, even high. BUT what is going to hurt him is the lack of strength. Even IJ is a good bit stronger than Collins. I think teams will look at him as a guarantee 1st year in Dev-league player. That is NOT what they want to invest a high pick on. So for him I think best case he goes in mid-20's. But with another year under his belt and in the weight room, I think he could move all the way into the top 10 just by waiting 1 year. That is a huge leap and much bigger paycheck just by waiting 1 year.
So I would give both say a 50% chance to return, which if their decisions are independent of each other would mean 25% chance both return, 50% change 1 of the 2 return, and 25% chance neither return.