Ko went straight into the Scottish Open on the back of her bronze medal at the Tokyo Olympics as she prepares for the British Open next week less than an hour away in Carnoustie.
Her best finish at the British Open was tied for third in 2015 after twice being the leading amateur in 2012 and 2013. Ko has already had two top six finishes at majors this year heading into the final major of the year.
"I actually haven't played well at The Scottish Open before, so definitely nice to be able to kind of finish on a high. The wind was a lot calmer today. So made it a little bit more gettable. I think I played really aggressive. Well, I tried to stay pretty aggressive and that's kind of been the mindset feeding off last week. [I] kept it to that and finish this week on a high and good momentum going into next week," Ko said.
O'Toole began the day tied for the lead at nine-under with Ariya Jutanugarn and Charley Hull, and the American took the lead with birdies on three of her first four holes.
Thitikul drew even with O'Toole with a birdie on the par-3 sixth, but O'Toole got back in front with a birdie on the par-4 ninth to turn in 31. She maintained her advantage throughout the back nine and finished with eight birdies, including all four par-5s.
"I tried not to look at the leaderboard," O'Toole said. "I tried to just trust in my caddie to guide me to whether we needed to lay or get aggressive, stay patient or whatnot. It wasn't until 18 that I saw that Lydia was not far off and I could tell when I made the birdie on 17 that there must have been some sort of gap because people were cheering, and I was like, OK, clearly I can't mess this up too badly now."
O'Toole was working with a new caddie, Michael Curry, after her previous looper retired following the Evian Championship. She said she had also been considering retirement.
"I'm getting married in December, and OK, my clock's ticking. I want to have kids. Like, how much longer am I going to be out here? I thought maybe this year would be my last year," O'Toole said. "I've never wanted to be a mum on tour as far as having a kid and doing all that. It doesn't sound fun to me. But at the same time, I don't know if I could stop playing golf now."
On the European Tour, Kiwi Ryan Fox finished in a share of 13th at the Cazoo Classic in Kent, England following a two-under 70 in his final round. Scotland's Calum Hill won the tournament for his maiden title.
