The exception proves the rule.
Aaron Donalds don't grow on trees.
Here you go:
The 3-technique is the most glamorous of all the interior alignments. By definition a 3-tech (or under tackle) is any player aligned on the outside shoulder of a guard. What makes it such a coveted position? He generally has free rein to get upfield and create disruption. Unlike a 0- or 1-, the 3-tech is tougher to double team simply by alignment and therefore can make more plays on ball carriers in the running game and get to the quarterback faster on pass plays.
The reason they face fewer double teams is that in a typical under front, the 3-technique aligns to the weak side with a defensive end outside of the weakside tackle. This means the tackle can’t double down and the center has a long way to go to try and execute a double team.
The one quality most associated with the 3-technique position is quickness. Defensive coordinators want their 3-techniques playing in the offensive backfield and there is no better way to do this than with a swift first step.
The heights of 3-techniques vary wildly across the board, although shorter tackles are often pigeonholed as 3's. The one thing they do have in common, though, is weight. It’s rare to see one tip the scales at over 300 pounds as holding up to double teams cedes priority to agility.
Sometimes fans get a little too caught up in weight. Leverage, technique, hands all have more to do with how well you play a position than weight does.
Odighizuwa is a lightweight DT at 285, compared to Gallimore at more than 300. However, Osa is the better run defender (at least heading into this season). The difference is that Osa knows how to maximize what he's got while Gallimore uses his quickness to win and doesn't focus as much on his technique (same with Hill, but Hill's technique is much worse).
Osa understands leverage, which negates an OL's size advantage. He stays low and keeps his weight centered so that he can uproot the lineman. No matter how heavy you are, if you lose the leverage battle, you are going to have trouble maintaining your position. However, Osa also makes good use of his hands to help with the leverage and keep OLs from getting into his body. This allows him to control the block. Him having long arms for his height and being a very good wrestler comes in handy here. Because of his technique, Osa can even play nose tackle in a pinch despite his weight. He received snaps there at UCLA.
Because of how he uses what he has against run blocking, he should be a good fit at the under-tackle in the hybrid 3-4, where he has one-on-one battles that will allow him to best use his skills. He doesn't need to be at the 5-tech or 1-tech because those positions are meant to draw double-teams and even with his skills, it's hard for a 285-pound player to hold his ground against doubles.