McIlroy looked set to miss his second major championship cut in succession on Friday.
The 24-year-old was five over par after 11 holes of his second round but birdied four of the next seven to add a 71 to his opening 69 and finish level par, nine behind clubhouse leader Jason Dufner.
McIlroy had bogeyed his first hole of the day, No. 10, before making double bogey on the par-3 15th and adding two more bogeys on Nos. 17 and 18.
"I was letting the round get away from me somewhat, but making four birdies on the last seven holes was nice to sort of redeem the round a little bit and keep myself in the tournament," said the world number three.
"It makes me feel good because maybe in the middle of the season or a couple of months ago I wouldn't have been standing up here. I would have been going home.
"It's good to be able to do that and fight back and makes you feel good about yourself going into the weekend. Walking up the second hole, which was my 11th hole today, all I wanted to do is be here for the weekend. All of sudden, I'm somewhat back in the tournament."
Graeme McDowell and Darren Clarke have also made the cut at Oak Hill.
McDowell is one shot ahead of McIlroy after hitting two rounds of 70 and 69, while Clarke sits two shots behind the world number three after shooting rounds of 69 and 73.