With one career victory among them, they upstaged more familiar names in the field at the Locust Hill Country Club, including last year's winner and world number one Yani Tseng, who struggled to a four-over-par 76.
Tseng, who felt tightness in her left wrist and forearm and received some massage treatment before her round, won last year's championship in Pittsford, New York by 10 strokes but found herself seven back after the opening round.
Recari, who recorded a victory in her rookie season on the LPGA Tour in 2010, posted her score among the early starters and saw it hold up with only fellow 25-year-old O'Toole and 32-year-old Sergas matching it.
By contrast with the other two, Sergas had a rollercoaster of a round, balancing three bogeys with three birdies through her first 14 holes before finishing strongly with birdies at the 15th, 16th and 18th holes to join them at three under par.
"They were silly bogeys," said the Italian said, who teed off in the last group of the day. "Like one bogey I was in the middle of the fairway 100 yards to the pin. So I was thinking, I was hoping, birdie and I ended up having a bogey.
"And another one, it was just a missed chip from the high rough around the green. So they were silly. They could have been par really."
Seven players were tied for fourth place just one shot back, including 2010 champion Cristie Kerr of the United States who won at Locust Hill by a massive 12 strokes.
Kerr said the heavy rough grown was a factor in keeping the scores modest. She and Tseng both posted totals of 19 under par in their victories here the last two years.
"I think it is going to play tougher," Kerr said. "The rough is measurably worse than the year that I won. It's just a lot thicker."
"It was very long that year, but it wasn't like really dense. I mean even with sand wedges out, it's a lot tougher.
"It's just gobbling up the golf balls this year."
Joining Kerr at two-under-par 70 were American Paula Creamer, South Korean world number two Choi Na-yeon and her compatriots Pak Se-ri and Jang Jeong, as well as Japanese Ai Miyazato and Mika Miyazato.
Six more players were another shot back at 71, two off the pace, including former winner Suzann Pettersen of Norway.
The 34-year-old Pak, who injured her shoulder in a freak fall earlier this year, was surprised to be in the hunt after only two weeks playing with her full swing.
"I never expect that's going to be solid round today," three-time LPGA Championship winner Pak said. "The last two days in practice were so-so.
"Low expectations help a lot."
One newcomer to the pro circuit received encouragement from a 14-times Grand Slam winner.
Cheyenne Woods, in her first tournament round as a professional, shot a three-over 75 after getting some encouraging text messages from her uncle, Tiger Woods.
"(He said) Just trust my abilities, have fun and be patient," she said.