Under her single name of Simon, Buhai represented South Africa as an amateur in the Women's World Cup of golf.
She turned professional at 18 and almost immediately won the 2007 Catalonia Ladies Masters on the European Tour.
Since then, she has added four more titles, including last year's South African Women's Open, and now competes mainly on the US-based LPGA Tour.
Her major record is poor, however, with only one top 30.
But an eight-birdie round was the ideal platform for a title challenge a she ended the first round on a seven-under 65.
"I got off to a great start, lipping out for birdie at the first and then making gains at two and three," said the 30-year-old Buhai.
"It was very solid all round and I struck the ball well all day. But there is an awful long way to go."
Park Sung-hyun lost her world number one ranking on Monday, but she underlined her determination to snatch it back after an opening 67.
On five under par, the South Korean was just two shots behind Buhai and one back from Japan's Hinako Shibuno and Danielle Kang of the United States.
Park had a chance to win a third major last week at the Evian Championship in France.
However, she slipped back with a closing 75 to finish tied sixth behind new world number one Ko Jin-young.
A six birdie round got the winner of the 2017 US Women's Open and 2018 LPGA Championship firmly back on the major track.
"I was a little worried coming in to this week at having to play back-to-back majors," admitted the 25-year-old.
"But it has been better than I thought. I've been able to focus and I am feeling good. My tee time was 7am and it's a while since I played so early, but I felt very comfortable on the course."
Shibuno felt quite at home on the English parkland lay out.
"Woburn is very similar to Japanese courses," she suggested.
"I was relaxed and confident. But I am very surprised at my position. My game was very good, and tomorrow I'm going to be playing and smiling all the time."
Kang, a US Solheim Cup player, had seven birdies, and also expressed her love of Woburn.
"It's a really gorgeous course, a lot different than playing a British Open on a links," she suggested.
"I always enjoy the challenge of the majors and I am delighted with my start. I have been working hard on my wedge play, and it paid dividends today."
England's Georgia Hall, the defending champion, and Kim In-kyung, the 2017 winner, were both on the same mark of three under par.
Ko stayed in touch for back-to-back majors with a 68 to be three off the lead.
The South Korean had a blistering run of four birdies in a row from the fifth.
But a double-bogey at the short 14th was a body blow.
Thailand's Ariya Jutanugarn, winner at Woburn three years ago, was also on 68 while older sister, Moriya, was one better.
Leading first round scores on Thursday in the Women's British Open at Woburn (par 72):
65 - Ashleigh Buhai (RSA)
66 - Hinako Shibuno (JPN), Danielle Kang (USA)
67 - Park Sung-hyun (KOR), Charley Hull (GBR), Moriya Jutanugarn (THA), Megan Khang (USA)
68 - Lee6 Jeong-eun (KOR), Ariya Jutanugarn (THA), Ko Jin-young (KOR)
69 - Kim In-kyung (KOR), Marina Alex (USA), Caroline Masson (GER), Jenny Shin (KOR), Georgia Hall (GBR), Brooke Henderson (CAN), Lizette Salas (USA), Carlota Ciganda (ESP), Morgan Pressel (USA), Agathe Sauzon (FRA)
70 - Minami Katsu (JPN), Kim Sei-young (KOR), Linnea Strom (SWE), Bronte Law (GBR), Ayako Uehara (JPN), Nelly Korda (USA), Sakura Yokomine (JPN), Chun In-gee (KOR) Kang Haej (KOR), Lindy Duncan (USA), Lee Jeong-eun (KOR)