John Sickles recently put up a list of sleeper prospects here. It was a fairly substantial list but not a huge one. I thought I'd take a quick look at the two O's he marked. The two Baltimore sleepers are Brandon Cooney and Ashur Tolliver.
Brandon Cooney: Started 10 games in 2007 at the rookie level with one relief appearance. He had a so-so year but with a very high Babip (Batted Balls in Play). As a starter he had a very strikeouts per nine innings rate of 6.11 (It's extremely hard to survive in the majors with a K/9 lower than 6) but with solid control 2-something. He became a fulltime reliever after that doing very well. He had a 13.15 k/9 in 2008 but with worse control at short-season A. 2009 he split time between A and advanced A but his k/9 innings was mid 7s but with good control. I don't see much coming out from him unlike Sickels but he definitely knows more than I do. If he's using a sinker it could just be a matter of having an improved defense behind him.
Ashur Tolliver: He signed in 2009 as an overslot signee in the fifth round making three scoreless appearances striking out 7 in eight innings of work. According to the scouting report he has an 88-93 fastball, a change-up and a slider. As a lefty the fastball is in the right range, so it should be at least average to above-average. It apparently hit 96 early in the college season which means he could become a second power lefty after Matusz. According to Baseball America his change-up is his best secondary pitch. He played for NAIA power Oklahoma City after transferring from Arkansas-Little Rock and posting an ERA of 2.17 in relief in the Cape Cod league. Apparently he's pretty small listed as 6 ft and 170 pounds. He will need to either improve or develop a good breaking ball if he wants to make it as a starter. Either way he could very easily end up in the top 10 next season with a power fastball and a good changeup and whatever he settles on as his breaking ball.