When Osuna misses, he doesn't miss the zone by much. I think Martin is helping him out quite a bit this game with pitches near the edges, but the stuff is undeniably good.
It looks like Osuna has three plus pitches right now. The change looks like a finishing pitch today, the slider looked like a finishing pitch the other day against New York, and the fastball has both velocity and some command right now.
I think they should be aggressive with him this season. The only issue with him potentially contributing to the big league team later on this year is that he doesn't have a whole lot of innings due to past injuries, so they shouldn't make his workload too large this year. Maybe they pull him out of the rotation in AA at some point in the summer and put him in the big league pen? That would give him experience, help the big team, but allow them to control/limit his innings and workload.
Maybe at some point you call up Osuna to relieve and then throw Castro into the big rotation to get innings (assuming you need a SP).