WRT Osa being a good 3-4 DE, the DEs you want in the classic 3-4 are not just big, they're also very tall or long. Think Spears. You want your 3-4 DE to line head-up on the tackles and keep the tackle out of their chest, so they can shed them and make run stops in 2 gaps. You don't want a DE who gets engulfed.
Osa could probably still play well as a 3-4 end, cause good football players are good football players. But he's a dude who would get engulfed. He'd have to make up for it as a pass rusher, and I bet he still could. But there's no doubt in my mind that he's a better 1 gap DL than 2 gap DL.
The holy grail of the old school 3-4 was the "pressure five" - the DE who could plug two gaps in run defense and also get to the the QB on passing plays. Those dudes were unicorns. They were never big sack guys, because they had to run straight through a tackle on their pass rush, but they could give the defense pressure without blitzing and freed up the rush LB even further. Most of your good 3-4s had one - dudes like Justin Smith with the Niners, Aaron Smith with the Steelers, Richard Seymour, or Haloti Ngata.