Pettersen shot an 8-under 64 to match Ochoa at 12-under 204 through three rounds at Big Horn Golf Club.
Ochoa, the defending champion in the event for the LPGA Tour's elite players, had a 69 that included a bogey 5 on the final hole. Pettersen already had finished her round, and thought she probably would be one shot off Ochoa's lead heading into the final day.
Pettersen missed a 15-footer for par on the second hole for her only bogey.
Pettersen, who began the day five strokes back, rolled in a 15-foot birdie putt on No. 1 for the first of her nine birdies.
Pettersen, the 26-year-old from Norway, made four birdie putts from 15-25 feet during her round, and tucked her approach shots near the pin for other birdies.
Last Sunday in Danville, Calif., Pettersen made an 8-foot birdie putt after Ochoa missed her try from 20 feet on the second playoff hole.
Ochoa, the LPGA Tour's best player over the past two years, reeled off three consecutive birdies beginning on No. 3 to begin her third round, then briefly opened a four-shot lead with a 5-foot birdie putt on the ninth hole that put her at 13 under.
But she played the back nine at 1-over 37, while Pettersen, who had a 33 at the turn, shot 31 over the last nine holes -- including a tap-in for birdie on No. 18.
Angela Park, who has clinched the 2007 rookie of the year award, shot a 69 and was tied with Jeon Jang (68) and Mi Hyun Kim (67) one shot back of the co-leaders.
Michelle Wie had her lowest round of the tournament, but that was a 5-over 77. She had 79s the first two days and remained in 20th and last place at 19-over 235, four shots worse than Bettina Hauert of Germany in the 19th spot.