Not really a great comparison. Even prior to vision issues with the rangers Olt came up to the majors and posted this .152/.250/.182 over 40 PAs which is remarkably similar to his current .153/.237/.372 over 156 PAs. The bigger worry is last year in AAA he hit .213/.317/.422 and .168/.276/.275. Ok so he had vision issues. In 2011 he had .267/.387/.504 at A+ as a 22 year old. While those numbers aren't terrible, he was about 2 years older than a lot of the players that end up there. Case in point, Almora turned 20 in April and is in A+. So, he should have arguably dominated the level of competition. Donaldson as a 22 year old in A+ hit .330/.391/.564. He was also far more consistent in his play in the minors. He had numerous minor seasons over .330 OBP where as Olt struggled quite a bit after A ball save for one really good AA season.
Additionally, Olt's glove is what's suppose to play big time and yet he's roughly 0 in UZR/150. Donaldson has been above 10 every year he's been at 3B. Also, Olt is getting murdered with k's. He currently sits at 35.3% k rate. Donaldson has never struck out that much at any level with a minimum of 50+ PAs.
Ultimately the problem becomes that Olt needs to be down in AAA. However, at this point Villanueva and Bryant are better prospects and Olt is 25. How long do you realistically hold out hope for him? Bryant likely will be in the majors before year end. If he plays well Olt's toast here. He can't play 1B because Rizzo is there and Bryant likely would be the better option at 3B. That then means you're trying to find time for him in AAA but as I said before, Villanueva's probably a better prospect. He could likely see time at 1B in Iowa but even then he hast to hit. And even if he does hit, you're talking about someone who will be 26 in August. As such, it's entirely possible he doesn't survive roster cuts at the end of the season.