After a back-and-forth opening 7 1/2 minutes, the Buckeyes (20-9) opened up the first double-digit lead for either team with a 7-0 run.
Samantha Prahalis started it with a deep three-pointer from the left wing, and she helped it continue by spoon feeding Martina Ellerbe for an easy 5-foot jumper from the baseline the next time down the floor.
Tayler Hill kept the spurt going when she stole a pass under the basket and led her own one-person fast break she completed with a pull-up jumper from close range to make it 17-12.
The Buckeyes battled back, though, digging in on defense to shut out the Hawkeyes for the last four minutes of the half and scratching out a pair of baskets to send the game to halftime knotted at 27.
Amber Stokes figured heavily in both baskets as she played in place of Prahalis, who spent the last 8:47 on the bench after picking up two fouls.
Stokes drove into the lane for a short jumper that drew the Buckeyes to within two, and two possessions later she skied for an offensive rebound that eventually led to Hill's game-tying jump shot.
Iowa center Morgan Johnson was the unlikely leading scorer in the first half with eight points as Ohio State chose to concentrate on stopping the talented perimeter trio of Wahlin, Jaime Printy and Kachine Alexander, who combined for 11.
Brittany Johnson led the Buckeyes with six points while Prahalis had five and Lavender, Stokes and Hill all had four.
Ohio State was guilty of 13 turnovers in the opening half, many coming as they tried to feed Lavender on the low block against a packed in zone defense.
Iowa, though, gave the ball up 11 times, and the Buckeyes were more resourceful when they did, finishing with an advantage of 14-7 in points off turnovers.
Ohio State dominated the first four-plus minutes of the second half. Lavender scored a pair of baskets to put the Buckeyes ahead four, and they played suffocating defense on the other end.
Adams blocked a pair of shots, and Iowa's only basket in the opening 8-2 sequence was via fast break after a turnover.
They eventually stretched the lead to 12 after a pair of Brittany Johnson three-pointers, but the Hawkeyes (22-8) were far from dead.
After the second Johnson trey, Iowa head coach Lisa Bluder called timeout with 10:04 to go, and her team rattled off the next 11 points as the Buckeyes committed four turnovers in five possessions.
To stop the bleeding, Hill took it to the basket and drew a foul on Morgan Johnson. Her pair of free throws restored a three-point advantage with 4:57 left, and 40 seconds later Ashley Adams stretched it to five with a layup set up by a great Prahalis pass.
Iowa was not about to quit, though, leaving the final heavy lifting to Brittany Johnson.
The Ohio State senior hit a trio of three-pointers in the final four minutes, all in answer to an Iowa basket on the Hawkeyes' previous possession.
"I was just, you know, in the zone, I guess," Johnson said. "I was focusing really hard. My teammates had confidence in me. Jantel threw me the ball after the rebound. She had confidence, and then Taylor and Sam threw me the ball. So I was just focusing on my shot."
The final one swished through the twine at the 1:16 mark and brought with it a sense that the Buckeyes, then leading 66-61, would move on to the semifinals.
"We have the utmost confidence in her shot," Lavender said. "And it just kind of went that way. It wasn't like a plan like, you know, they hit a three, we're going to hit them back with a three. It was like the play opened up that way. And I either got a rebound, passed it to Brittany, or we reversed the ball and hit her in the corner. But she did a great job focusing tonight, and that's what we needed to win."
The Buckeyes closed out the game by making 5 of 6 free throws to set the final margin.
Lavender added 11 points and 14 rebounds while Hill had 12 points and five assists. Prahalis had seven points and eight assists.
Printy led Iowa with 19 points, but the Hawkeyes' leading scorer on the season was held to eight field goal attempts as Ohio State worked hard to deny her the ball and keep her far from the basket when she did catch it.
Iowa shot 32.8 percent (19 for 58) against a Buckeye defense that has improved steadily down the stretch as Ohio State has put together a seven-game winning streak.
"I think they're playing better defensively, absolutely," Bluder said. "I just think they seem like they're playing more team defense. They'll help more. I think their on-ball pressure is a lot better, but they're just willing to help each other out and play a little bit better team defense than they were the first time that we played them."
Both games can be seen on the Big Ten Network.
Ohio State, the two-time defending tournament champions, swept the season series with the Spartans.