The Buckeyes (15-3, 6-1) trailed by as many as 12 points in regulation but forced the extra period, tied at 83.
Clark scored all of Iowa's nine overtime points. A 3-pointer from Rebeka Mikulasikova gave Ohio State a six-point lead with 1:14 left in overtime. A basket by McMahon and four late free throws from Jacy Sheldon sealed with win and fans streamed out of the stands to celebrate.
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Clark and a fan collided on the court, knocking her to the floor.
After the game, Clark said she wasn't seriously injured during the collision.
"Kinda scary. It could have caused a pretty serious injury for me," Clark told reports after the game. "It knocked the wind out of me. Luckily, my teammates picked me up and helped me off the court. [Ohio State's] [athletic director] already came and apologized to me, so I really appreciate that.
This is what comes with the territory. I'm sure they tried their best to do whatever they could. Obviously, it didn't work, and that's disappointing."
The Buckeyes' win snapped a 15-game win streak for the Hawkeyes (18-2, 7-1 Big Ten).
Iowa: Clark carries the load for Iowa, and it was never more apparent than in this game, when the next highest scorer was Molly Davis, who scored 14 points.
Ohio State: A signature win for the Buckeyes that should give them a big boost in the AP Top 25.
Iowa: Hosts Nebraska on Saturday.
Ohio State: At Illinois on Thursday.