Oh was in the lead after starting her round with 1-over, but coming back with four birdies and an eagle over her last seven holes. Late in the day Choi stormed to the top of the leaderboard, before losing her lead with a bogey on her final hole.
South Korea's Choi and South Korean-born Australian Oh were a shot clear of four players who shot 70, they were: Holly Clyburn and Charley Hull from England, Australia's Katherine Kirk Norway's Caroline Martens. Defending champion Cheyenne Woods shot 73.
Oh said: "I got off to a really slow start, and then it all kind of unfolded at the end. I was happy with the finish.
"I mean I bogeyed and I got a good up and down on the second, made a good putt on the third which got me down to one over and I just kind of told myself to get a few more birdies. I thought well you know, I don’t really know this nine as well. So maybe just try to get a couple more, and just finish around even. And just keep myself in contention a little bit and yeah I just got a lot more birdies than I thought."