In a tight match, Haas took an early 1-up lead when Oh bogeyed the second hole. However, Oh won the next to square the match as Haas bogeyed the third.
The match remained all square until No. 10. Haas stumbled to a double bogey to fall 1-down and Oh won the next with a par four to extend his lead to 2-up. Haas came back to birdie the par-3 13th to get within 1-down.
Oh responded with a birdie at the 14th to reclaim a 2-up lead. Oh's lead dropped to 1-up when he bogeyed the 15th though.
Oh's lead would again be 2-up when Haas bogeyed the par-3 16th. The duo parred the 17th and the match was Oh's. Oh will square off against Nick Flanagan in the semifinals.
The 19-year-old Flanagan bested 44-year-old Jerry Courville, 1-up. Courville a three-time quarterfinalist at this event grabbed an early lead when he birdied the par-4 first. Flanagan quickly evened the match as Courville bogeyed the second.
Flanagan owned the match from there. He won Nos. 3, 5 and 6 to grab a 3-up lead as Courville played those holes in four-over par.
Courville fought back to 1-down with wins at the seventh and ninth. The players rotated winning holes from the 10th to 14th with Flanagan winning the 14th to go 2-up.
Courville birdied the 17th to remain in the match, but Flanagan was victorious after the pair parred the 18th holes.
The other semifinal match will see Lee Williams face off against Casey Wittenberg, the youngest remaining competitor at the age of 18 years old.
Wittenberg downed George Zahringer, the 2002 U.S. Mid-Amateur champion, 1-up. Zahringer bogeyed two of his first three holes to fall 2-down but Wittenberg bogeyed the par-5 fourth and saw his lead slip to 1-up.
Zahringer birdied the seventh to square the match but Wittenberg birdied the next to reclaim the lead. The see-saw continued as Zahringer birdied the ninth to again square the match.
Wittenberg won the 10th, but Zahringer won the 12th and the pair each had a birdie over the next two holes to keep the match all square through 14. They then halved the 15th and 16th.
The turning point came at the 17th when Zahringer bogeyed the hole. Each man parred the last to give Wittenberg the win.
'It's obviously a huge goal of mine to be able to play well in the U.S. Amateur,' said the 18-year-old Wittenberg. 'It's our biggest tournament and a lot of people that are great players can't wait to get over the hurdle in the U.S. Amateur.'
Williams bogeyed the first two holes to trail Patrick Carter 2-down after two holes. Williams fought back to square the match with wins on Nos. 5 and 6.
The 21-year-old Williams also won the seventh to grab a 1-up lead. The duo split the ninth and 10th holes, before Williams won the 11th to regain a 2-up advantage. He extended the lead to 4-up with wins on Nos. 13 and 14. He won the match when both players parred the 15h.
'It's an honor to get here because there are so many good players that try to qualify,' said Williams. 'So when you get here, you might as well be thinking about winning.'
The U.S. Amateur is one of 13 national championships conducted annually by the United States Golf Association, with 10 of the events strictly for amateurs.