The top-ranked Mexican star, coming off her first major victory two weeks ago in the Women's British Open, had a 14-under 199 total on the Royal Mayfair course. She opened with rounds of 70 and 65 and hasn't made a bogey in 40 holes.
Paula Creamer (66) and Laura Diaz (68) were 10 under, Ya-Ni Tseng (67) followed at 8 under, and Juli Inkster (73) and Kimberly Hall (72) were 6 under. Shi Hyun Ahn, tied for the lead with Inkster after the second round, had a 74 to drop to 5 under.
Ochoa, a stroke behind Inkster and Ahn at the start of play Saturday, had five birdies in a front-nine 30 and added two more birdies on the back nine to match the course record set by Karrie Webb on Friday. Ochoa had a chance to break the record, but her 15-foot putt from the fringe on 18 stopped inches from the hole.
Alena Sharp topped the three Canadians left in the field, shooting a 67 for a 4-under 209 total. The Hamilton, Ontario, player had three birdies on the back nine.
'I'm full of adrenaline right now, playing the back nine and shooting 3 under. I need to make sure I get rest and I stop thinking about golf,' Sharp said. 'I felt like I didn't have a lot of pressure going into today because I was eight shots back, but shooting a good round today, maybe there's a little pressure for tomorrow.'
Nancy Harvey of Swift Current, Saskatchewan, was 2 over after a 70, and A.J. Eathorne of Penticton, British Columbia, was 4 over after a 72.
U.S. Women's Open champion Cristie Kerr, the winner last at London Hunt in Ontario, was 11 strokes back at 3 under after a 71.