Entering the extra frame, Oklahoma was shooting 89 percent from the free-throw line, but made only four of its eight foul-shot attempts in overtime.
Iowa State guard Jadda Buckley led all scorers with 29 points on nine of 10 shooting. Buckley made six of the Cyclones' 13 successful 3-point attempts.
Hook matched her career high with 26 points, and Campbell added 16 and nine rebounds.
“This was a measuring stick,” Fennelly said. “When you come here and win, you're doing something right.”
The loss dropped OU to 10-5 and 1-1 Big 12 play this season. OU returns to action Wednesday morning at Texas.
“We have just got to play hard and be hungry,” Campbell said. “There is nothing really to say. We just have to play hard from the get-go and we did not do that today.”