Ochoa, who has won the past two tournaments, threw up her arms after making a long birdie putt on the par-4 18th hole to finish the day with a 5-under 67.
Oh and Gustafson each had 66s on the course at Columbia Edgewater Country Club.
Ochoa won the Canadian Women's Open in Edmonton, Alberta, last week. It was her second straight victory and fifth of the season. Her previous victory at the Women's British Open was her first major title.
'I'm going to try very hard to get three in a row,' said Ochoa, who is from Mexico. 'That would be very special.'
Ochoa finished the day with six birdies and a bogey. She made a 25-foot putt from the fringe on the 18th.
'To be honest, I was confused,' she said. 'It's not that I hit the ball bad, I was just confused with the greens. I couldn't get it adjusted.'
The 19-year-old Oh, from South Korea, had never shot a round that low.
'It's the first time,' she smiled. 'I'm so very happy.'
Oh sparred a bit with caddie Doug Wilbur over the length of her seven birdie putts. She estimated one, on the par-5 10th hole, at three feet. But he said it was more like seven.
'That's how well she's putting,' Wilbur said. 'Everything looks so close.'
Leta Lindley and Angela Stanford were two shots back of the leaders with 68s.
Michelle Wie, playing in Portland on a sponsor's exemption, continued her struggles this season with a 79. Wie has made the cut just once in four previous appearances this year on the tour.
The 17-year-old has had trouble with her left wrist, but played down the injury's impact on her round, saying she was a bit sore 'but not bad.'
Defending champion Pat Hurst was 5 over on the 6,377-yard course near the Portland airport, where temperatures climbed into the mid-80s on Friday.
The biggest gallery followed a group that included Hall of Famers Juli Inkster, Karrie Webb and Nancy Lopez.
Webb had the best round of the group with a 70, while Inkster shot a 75 and Lopez was among those in last place with an 82.
Lopez, who has 48 tour victories, is playing in her fifth tournament in a comeback bid. She has yet to make a cut. But she has won three times in Portland over her career.
Ochoa also had a large contingent of fans in her gallery, including a pair that carried a large Mexican flag. A sponsor's group of some 150 fans traveled from Guadalajara to watch her.
'It was like playing at home,' she said.