Lenzelle Smith, Jr., added 12 points — all in the first half — with Sam Thompson getting 11 and Amir Williams had a career-high 10 for OSU (18-6, 8-4 Big Ten).
Pesky Northwestern (13-12, 4-8) led throughout the second half despite having only seven scholarship players because of injuries.
Tre Demps led the Wildcats with 16 points, Reggie Hearn had 12, and Dave Sobolewski 10.
No one would have been surprised to see the Buckeyes blow out the Wildcats. They held a 111-45 upper hand all-time against Northwestern, including 65-14 in home games.
Also, the Wildcats were wounded. They recently lost starter Jared Swopshire, who underwent arthroscopic surgery on his right knee after being hurt Feb. 9 at Iowa while Drew Crawford has missed the entire season with a shoulder injury. Also sidelined are Sanjay Lumpkin (wrist), 7-foot-2 Chier Ajou (knee), and 7-foot Alex Olah (upper body).
The Wildcats led by four points for several minutes down the stretch. They led 54-52 on a three-point play by Kale Abrahamson, who had driven the left baseline and then tossed in the layup while being fouled with 5:15 left.
Ohio State countered with Thomas, who had struggled with his shot and with sticky defensive coverage all night, getting fouled after he muscled in a bucket. His three-point play put the Buckeyes ahead 55-54. The lead grew to three when, after a Northwestern miss, Thompson scored on a layup.
Demps, who was 4 of 6 behind the arc, drilled one from the right wing to tie it again with 3:24 remaining as an uneasy crowd of 15,842 at Value City Arena took a break.
And the Buckeyes took over.
Thomas scored in traffic and drew the fifth foul on Abrahamson with 3:03 left.
Hearn drove from the left side and his shot, which could have tied it, instead sat on the back of the iron for a full two seconds before finally falling.
Aaron Craft, who had seven points, three rebounds, and six assists, drove through the heart of the Northwestern zone and, shifting hands in mid-air, flipped in a left-handed layup to push the lead to 61-57 with 1:52 remaining.
The Wildcats, who had frustrated OSU's offense all night, could not find the range. After another miss, Thomas was fouled and banged in both free throws. Thomas then added baskets in the final minute before Thomas made two more foul shots to complete the 12-point assault.