"We're winning this," he said.
Mills said he preached confidence to the 15-seed Golden Eagles, who beat the Buckeyes 75-72 for their first NCAA tournament win since 1974, when they were known as the Titans.
Few expected an Oral Roberts victory.
In ESPN's Men's Tournament Challenge, more than 95% of the brackets (nearly 14 million) picked the Buckeyes, who were 16-point favorites. It was the biggest upset since UMBC's first-round victory over top-seeded Virginia in 2018 as a plus-20.5 underdog and the eighth biggest since 1985.
The loss punctuated a season of turbulence for an Ohio State team that had positioned itself to compete for a No. 1 seed at one point in the season. The Buckeyes entered the month of February with four top-25 road wins. By Feb. 18, Chris Holtmann's squad had amassed an 18-4 record, but the Buckeyes lost the next four games before making a run to the Big Ten tournament title game, where they lost an overtime thriller to Illinois.
"I want to first begin by giving Oral Roberts credit," Holtmann said after the game. "I thought they really played well, as we expected they would. I thought their two players that obviously concerned us were terrific. So give them, their coaches, their players, their program, the university a lot of credit for this win."
Max Abmas, the nation's leading scorer, finished with 29 points (10-for-24, 5-for-10 from the 3-point line) and Kevin Obanor had 30 points and 11 rebounds in the biggest upset of the NCAA tournament thus far.
"We're very confident," Obanor said after the game. "When they rank us, it's only just a number at the end of the day. We put our shoes on just like they put their shoes on. ... We just knew that we were the underdogs and we wanted to win for our brothers."
Overall, the Buckeyes finished 3-6 in their last nine games of the season. Both Justin Ahrens and Duane Washington Jr. missed potential game-tying 3-pointers in the final seconds of overtime. Ohio State committed 16 turnovers. Oral Roberts had six.
The Buckeyes could not handle Abmas and Obanor, who combined to shoot 7-for-16 on contested 3-pointers, compared to Ohio State, which finished 1-for-10 on the same shots, per ESPN Stats & Information research.
But Mills did not want to dwell on the victory. He said he appreciated the historic nature of the game but also wanted his team to prepare for Sunday's game against Florida, which advanced to the second round after beating Virginia Tech on Friday.
He said he would advise his team to avoid returning any nonessential text messages until Monday.
"I thought we would win," Mills said. "Our guys thought we would win. The reality is you're about to turn around and play another one. So your celebration had better be pretty quick."