Ochoa, who has won two straight tournaments, had seven birdies earlier in her round that helped her finish with a 66, giving her an 11-under 133 after two rounds at Columbia Edgewater County Club.
Gustafson, who shared the lead going into Saturday's round with rookie Ji-Young Oh, had a 68 to go to 10-under 134.
Ochoa won the Canadian Women's Open in Edmonton, Alberta, last week. It was her second straight victory and fifth of the season. Her victory at the Women's British Open was her first major title.
After she bogeyed the par-4 17th hole, Ochoa's tee shot went into the trees on the par-4 18th. With her second shot into the rough in front of the green, Ochoa chipped within less than 10 feet -- but her putt lipped out for the bogey.
Ochoa, who is from Mexico, said she was playing aggressively for a low round.
'I'm just happy about being in good position,' Ochoa said. 'And I'm ready.'
Sophie Gustafson, who is from Sweden, has not won on the LPGA Tour since 2003. Her best finish this season was a tie for fifth at the Safeway International. She had five birdies and a bogey on Saturday.
'I was just trying to keep up with Lorena,' Gustafson said. 'She got off to such a good start. I was just trying to hang on.'
The 19-year-old Oh, from South Korea, was four shots back of Ochoa after a second-round 71 that gave her a 7-under 137 going into Sunday's final. Mhairi McKay of Scotland had a 67 and was also at 7-under.
Natalie Gulbis, who won her first tournament title earlier this year at the Evian Masters, finished with a 68 for a 5-under 139 on the 6,377-yard course near the Portland airport. Diana D'Alessio (69) also was 5 under.
Michelle Wie, playing in Portland on a sponsor's exemption, continued her woes this season with a 75 to go 10 over, well above the projected cut. Wie, who has had trouble with her left wrist, has made the cut just once in five appearances this year on the tour.
Nancy Lopez, who has 48 tour victories, had a disastrous 91 on Saturday, putting her 29 over after her two rounds.
This was her fifth tournament in a comeback bid. She has yet to make a cut.
She didn't put up Lopez's score. But golf's gracious ambassador was smiling as she went to the scorer's tent after her round. A group of fans chanted 'Nancy! Nancy!' when she emerged.
'I just have to forget today,' she said. 'It was brutal.'