'I can't even describe it,' said Prammanasudh, who collected $150,000 for the win. 'I worked so hard for this. There were tough competitors out there. I'm just glad I finished on top.'
Cristie Kerr (68), Christina Kim (68) and Catriona Matthew (71) shared third place at minus-9.
Prammanasudh came into the final round with a two-shot advantage over Ochoa, but the two were knotted in the lead quickly on Sunday. Ochoa tapped in for a short birdie putt at the second, then Prammanasudh three-putted the third green to fall into a tie for the lead at minus-10.
The sixth hole proved to be critical on Sunday. Prammanasudh rolled in a 35- footer for birdie, while Ochoa missed a 15-foot par putt. Prammanasudh's lead was two, but she padded the edge at the par-5 seventh. She chipped her third inside a foot and tapped in for a three-shot lead, which she took to the turn.
Ochoa cut into the lead at the 13th when her 12-foot birdie putt fell into the cup. She had a great chance at birdie at the 15th, but her 5-foot try moved right at the end.
Prammanasudh held a two-shot lead as the pair played the par-3 16th. Ochoa hit her tee ball 30 feet from the hole, while Prammanasudh was half that distance. Ochoa's birdie putt came up short, but Prammanasudh sank her putt for a three-shot advantage.
Ochoa could not use her length to her advantage on the short, par-4 17th. She only made par, but Prammanasudh hit her second close. She had 87 yards to the pin and knocked her approach past the hole. The spin brought the ball back to three feet and she converted the short putt for a four-shot margin with one to play.
After a long wait on the par-5 18th tee, Ochoa finally made a putt on the back nine. She rolled in a 15-footer for birdie at the last and Prammanasudh two-putted for a routine par and her first LPGA Tour victory since joining the circuit in 2003.
'I feel great. I don't know if I really believe it yet,' admitted Prammanasudh. 'To be able to hold off a competitor like Lorena and Catriona and some of those bigger names out there is special.'
Ochoa collected her third runner-up finish this year.
'I wanted to be the champion, but I'm happy for her,' said Ochoa. 'I'm happy with my second place. She didn't let me breathe. It was pretty exciting until the 16th.'
Rachel Hetherington (67), Karrie Webb (71) and Heather Bowie (72) shared sixth place at 4-under-par 284.
Jeong Jang struggled to a 2-over 74 in the final round and finished alone in ninth place at minus-3. Grace Park fared worse than Jang on Sunday. Park posted a 3-over 75 and took a solo 10th at minus-2.