Olesh was committed to Michigan since July 8.
The Nittany Lions faced two top 25 passing offenses earlier this season and allowed a combined 301 yards.
The Nittany Lions will face No. 1 Oregon in the Big Ten Championship game on Saturday.
Kortovich's commitment came just after Purdue fired its head coach.
The junior back had a rough five-game mid-season stretch. Now, he's back to 100% and is ready for a conference title.
The Nittany Lions became 7-1 for the first time since 2019.