I don't think he looks like a good fit along-side Sullinger, though.
Why?
I mean having a defensive minded, long big would make a more rounded duo, but a quality passer, guy who can run the floor, put it on the floor, hit an open jumper would make for a great offensive combo.
And we already have Melo in line to fit the defensive minded big prospect slot.
Olnyk doesn't play great horizontal, high-paint defense and (granted, this is just mho) I don't think you could coach him to become that guy. I think he has good enough feet and definitely the size to become an adequate low-post defender with coaching. But Sully already is an extremely good low-post, baseline defender. What you need next to him is for another big with complementary skills.
That's why Sully fits so well with Garnett. Garnett is one of the greatest horizontal, high-paint defenders ever. So he and Sully complement each other on defense very well. They also complement each other on offense, too.
I do agree that Olnyk, as a stretch big who can run would be a good complement to Sully on offense. But not on defense.
I'll offer the caveat that players don't always get the best coaching and development through college to really bring out their skills. Just because they haven't exhibited much ability at something in college doesn't mean it's not there. It is not beyond the possibility that Olnyk has the core skills that, given proper development, couldn't turn him into a better defender of passing and driving lanes. That's the kind of thing that Danny and his staff are paid to figure out.
But from the little I've seen, I'm not expecting him to become an all-world defender.