Korda hadn't played since beating Lydia Ko in a play-off to win the Drive On Championship in Florida seven weeks ago.
But after shaking off some rust early in the week she battled through the "brutal" windy conditions at Palos Verdes Golf Club to emerge with the 10th LPGA title of her career.
Korda led by three after an eagle at the 14th but closed with back-to-back bogeys in a two-under-par 69 to fall into a tie on nine-under par 275 with fellow American O'Toole, whose five-under 66 was the lowest round of the day.
The cold wind whipping across the scenic Palos Verdes peninsula south of Los Angeles made for a chaotic final round, the gusts moving the balls on some greens and wafting good shots off target.
Australian Gabriela Ruffels and American Alison Lee both held a share of the lead on the front nine before finishing tied for second on 276, Ruffels closing with a one-under 70 and Lee a one-over 72.
Overnight co-leader Shin Ji-yai of South Korea, a former world number one and two-time major winner, was making a rare LPGA appearance on a sponsor's invitation in the tournament hosted by her childhood idol Pak Se-ri.
She carded a two-over 73 to join a group on 277 that also included Thailand's Jasmine Suwannapura (70) and American Andrea Lee (68).
"The weekend was brutal," Korda said. "Today was just really tough. But I think that's also the beauty of the game.
"It's fun to play in these conditions and kind of be a little bit more artistic out here."
Korda, one-under after two birdies and a bogey on the front nine, gained valuable ground with a rare birdie at the par-four 12th, where she fired out of the rough to about 12 feet and rolled in the putt to join a tie for the lead on nine-under.
The reigning Olympic champion was alone atop the leaderboard by the time she arrived at the par-five 14, where she reached the green in two and made a testing left-to-right downhill putt for eagle that gave her a three-shot lead on 11-under.
Normal Nelly things
A bogey followed at 15 - where she watched the wind push her ball off the green. Unable to convert a long eagle effort from the fringe, she settled for birdie at the par-five 16th, but she bogeyed 17 and 18.
"Once I made the eagle, I got maybe a little nervous where I kind of got a little ahead of myself and started making some mistakes," Korda said.
"I seem to always make it interesting ... just doing normal Nelly things, making it interesting."
O'Toole, meanwhile, was finishing strong, her birdies at 16 and 17 keeping her within striking distance and a long par-saving putt at the last enough to put her in a play-off.
"I can't be more proud to be in this position," said O'Toole, who had five birdies and an eagle along with two bogeys in her round.
"That was actually my first play-off ever, so I really enjoyed it. I came from behind and gave it my best chance. I can't be more proud."
O'Toole, who claimed her only LPGA title at the Women's Scottish Open in 2021, said she realized she could be headed to a play-off when Korda's ball raced through the green at 18.
"That's a really hard up and down," she said.
