An intimate gathering of 533 turned out at Welsh-Ryan Arena to watch the 24th-ranked Buckeyes' 19-game series winning streak snapped.
In Evanston, the Buckeyes held a 13-6 lead after Samantha Prahalis' jumper with 9:14 to go, but little went right for the visitors after that.
They were outscored 18-3 the rest of the half as their offense abandoned them.
Ohio State made only 1 of 15 field goals in the next nine minutes, letting a solid defensiv effort go for naught.
The Buckeyes pushed the tempo but seemed out of sync, rushing shots in an effort to put their recent doldrums behind them, but that may have made things worse.
Ohio State held Northwestern to 36 percent shooting (9 for 25) from the floor in the first half but found itself down 23-16 because of an abysmal 7-for-34 (20.6 percent) half of its own.
The worst was yet to come. While the offense never really improved, the Buckeyes' defense came and went in the second half.
The Wildcats (14-4, 3-2) led by as many as 20 and were up double-digits for most of the final 20 minutes. The closest Ohio State got was seven when reserve Amber Stokes made a steal and went coast-to-coast for a layup with 2:19 left.
Northwestern struck right back, however, as sophomore forward Kendall Hackney slipped behind the Ohio State full-court press for a layup nine seconds later.
The Buckeyes failed to capitalize on multiple opportunities to cut into the lead over the next minute until they finally cut it to seven again when Prahalis followed a missed free throw with a layup at the 1:07 mark.
That was too little, too late as Northwestern's Beth Marshall made a pair of free throws and then teammate Brittany Orban made two more to ice the game.
The Buckeyes shot 31.5 percent from the floor for the game and went 4 for 22 from three-point range. To make matters worse, they made only 3 of 12 free throws.
Prahalis had 19 points as well but no one else had more than six for the Scarlet and Gray.
Hill had been the most consistent player during the rough patch that has seen the Buckeyes drop five of their last seven games, but she missed all 5 shots she took from the field and had one point. She was 1 for 6 from the foul line.
Orban, a junior from North Canton (Ohio) Hoover, had 13 points for NU while Hackney of Cincinnati Mount Notre Dame had 11.