O'Reilly's winner came shortly after a sequence during which she headed a ball off the crossbar -- and then struck the rebound with a header off the same spot. Her goal came in the 72nd minute when she tapped Natasha Kai's cross into an empty net.
The Koreans' best chance came in the 13th minute when Cha Yun Hee received a through ball from Park Hee Young and blasted a shot off the top of the left post.
The USA closed out a three-game series with South Korea with a 2-0-1 record.
"Of these three games, I think this was the best one," U.S. coach Pia Sundhage said. "They are a very good team. Tonight though we were back to a wide field that really helped us a lot and it helped us score a great goal."
Nov. 8 in Tampa
USA 1 South Korea 0. Goals: O'Reilly 72.
USA -- Barnhart, Chalupny (Cox, 55), Rampone, Markgraf (Buehler, 46), Ellertson (Mitts, 46), Wagner (Kai, 55), Lloyd, Boxx, Tarpley (White, 46), Hucles, O'Reilly (Dalmy, 74)
South Korea -- Jun (Kim Jung Mi, 46), Yi (Hwang, 41), Kim Do Yeaon, Kim Yoo Mi, Cho, Kim Sooyun, Kwon, Lee Jang Mi, Lee Eun Mi (Kim Huiyeong, 25; Jeon, 46), Cha, Park (Han, 71)
Referee: Meredith Hackett (USA).