But the Hornets, who have won 11 straight, finished the half on a 13-3 run to take command.
OCC, which shot 32.7% from the field, did not record a field goal the final 5:14 of the first half and the first 1:55 of the second, trailed, 53-34, with 12:29 left in the game.
OCC never got closer than 13 and are forced to settle for second place.
The Pirates have won five of their six one-point games this season and all three of their overtime contests, which will result in the 20th postseason appearance in Thornton's 22 seasons as coach.
But a third conference title in the last five years just wasn't in the cards for OCC.
"I never really felt comfortable with the game," Thornton said after Fullerton, the state runner-up last season, finished shooting 49.1% from the field, had three fewer turnovers than OCC and won the rebounding battle, 44-33.
"Defensively, they took us out of everything we wanted to do. They pressured us and we couldn't get into anything. And, defensively for us, there were too many holes in that [three-two] zone.
"I knew we had to play really well just to be in the game. I thought [the Hornets] played really well tonight. And when they're playing really well, we're going to have to have our A-plus game to be successful. We didn't have our A-plus game tonight."
OCC sophomore Samantha Hansen, an all-conference player last season who came in averaging 16.4 points and 7.2 rebounds, was frustrated by Fullerton's depth, quickness and length. Hansen scored OCC's first field goal, but had only one other bucket until the final 1:35. She finished with eight points and a team-best five rebounds. She was three for 10 from the field.