OSA is not good depth in this scheme. When he isn't a true 3 tech shooting a single gap, he is often engulfed by bigger blockers and almost always wiped by double teams. Technically, he is a DE in this scheme, but the 5 man line requires big DE's. Osa cannot play NT. The DE has to shoot the gap between the guard and tackle, but he is also responsible for helping the gap beside his, so taking on a block and sliding a bit if the play is in 1 gap over.
Osa's only real value would be in all-out pass rush, so he could come in on third downs, but he is paid starter's money. He is no longer a starter. Even in a 4-3, last year the starters were Williams and Clark who are bigger and better than Osa. He ended up only playing 40% of the defensive snaps. Bottom line....he is overpaid as a back-up rotational guy. So they signed another big body guy who can also stretch outside (he played out on the OT's against us last year), and they will most likely draft another player.
Winfrey, Osa, and even Solomon Thomas were all released or traded because they don't fit the scheme. Trust the defensive coaches. With all the leverage Jerry has given the coaches this offseason, there's no way he is making these moves, especially the Osa move, without the coaches telling him they can't use these guys.