I agree with Risen, depending on the deal Osa wants. Definitely want him back if he signs a deal with low guarantees. If he's trying to get that "good" market value deal let him walk.
Advanced stats say he's good. However, not sure his splash plays(Sturm terminology stuck in my head) is worth a upper level market value extension.
Cowboys are in a tough spot cause outside of Osa, the rest of the DT's suck. Mazi hasn't really shown much to be counted on and Rogers is a 1 tech DT that spent time on the Bengals PS squad and rode the inactive bench here.
A rookie DT would contribute, due to the lack of depth/talent but agree it could be at least 1 year to get a more polished rook DT at a NFL consistent level.