On 12 under par, Buhai led by three shots going into the weekend's final two rounds.
Japan's Hinako Shibuno was the nearest challenger on nine under after a 69, with American Lizette Salas on eight under after a fine 67.
Buhai led by one overnight and she achieved her goal of reaching double digits with a flawless five-birdie round, including a blistering run of four in six holes from the 11th.
Sally Little, at the 1988 du Maurier Classic, is the only South African to have won a women's major and Buhai is eager to follow in her footsteps.
"I had a slowish start today, but put my foot down on the back nine and I'm feeling good," said the 30-year-old.
"I'm trying not to think about it being a major. Just want to stay doing what I'm doing."
The local crowd had plenty cause for optimism with three English youngsters inside the top 10.
Home club member Charley Hull (70) and Bronte Law (67) were on the same seven under mark, with defending champion Georgia Hall just one shot further back after a second successive 69.
The trio have been friends for years, all featuring in the same English Under-13 squads when they had dreams of turning professional and playing in the British Open.
"I used to stay with Charley and play golf a lot when we were very young," Hall said.
"I remember her Dad made the best breakfasts. The three of us all played in England teams together and it is nice to have long time pals on Tour. We all motivate each other, spur each other on."
Hull has extra pressure as the club member, and she slipped up with back to back bogeys at the 13th and 14th.
But she showed her maturing attitude with birdies at 15 and 16.
Law claimed her first LPGA title at the Pure Silk Championship in May.
Now she seems set to make her Solheim Cup debut alongside Hull and Hall at Gleneagles next month.
Lurking on the leaderboard are the world's top two from South Korea.
Park Sung-hyun had a 70 for seven under and the top ranked player, Ko Jin-young, was one behind on six under after shooting the same score.
Ko won the Evian Championship last week, and is aiming for a third major title this year.
"I think I am in a really good position going into the weekend," she said.
But she lost a bet with her caddie, Dave Brooker.
"He said he would give me 100 dollars if I made five birdies. But I only got to three so I have to give it to him," she said.
Lydia Ko, who became the world No.1 as a teenager in 2015, continued her dismal run of form.
She missed the cut by six shots in last week's Evian Championship.
Rounds of 76 and 80 left her on 12 over par and all but propping up the field and facing another free weekend at Woburn with the cut on one over par.
Leading second round scores in the Women's British Open at Woburn on Friday (par 72):
132 - Ashleigh Buhai (RSA) 65, 67
135 - Hinako Shibonu (JPN) 66, 69
136 - Lizette Salas (USA) 69, 67
137 - Bronte Law (GBR) 70, 67, Celine Boutier (FRA) 71, 66, Park Sung-hyun (KOR) 67, 70, Charley Hull (GBR) 67, 70, Caroline Masson (GER) 69, 68
138 - Carlota Ciganda (ESP) 69, 69, Georgia Hall (GBR) 69, 69, Ko Jin-young (KOR) 68, 70, Ariya Jutanugarn (THA) 68, 70, Danielle Kang (USA) 66, 72
139 - Nelly Korda (USA) 70, 69, Lee Jeongeun (KOR) 68, 71, Marina Alex (USA) 69, 70, Minjee Lee (AUS) 71, 68
140 - Brooke Henderson (CAN) 69, 71, Morgan Pressel (USA) 69, 71, Pavarisa Yoktuan (THA) 73, 67, Kim In-kyung (KOR) 69, 71, Olivia Cowan (GER) 73, 67
141 - Megan Khang (USA) 67, 74, Kim Hyo-joo (KOR) 71, 70, Yan Jing (CHN) 71, 70, Lee Jeongeun (KOR) 70, 71, Anna Nordqvist (SWE) 71, 70, Lexi Thompson (USA) 71, 70, Moriya Jutanugarn (THA) 67, 74, Athaya Thitikul (THA) 73, 68