Ionescu, a 5-foot-10 freshman, finished with 11 points, 10 rebounds and 10 assists. Ruthy Hebard, also a freshman, scored 15 and grabbed 10 boards for her ninth double-double of the season and Maite Cazorla had 13 points.
Hebard's layup opened the scoring and the Ducks (16-9, 6-7 Pac 12) never trailed. UCLA's Monique Billings hit two free throws to make it 4-all, but the Bruins went without a field goal for nearly four minutes and Bando scored Oregon's final seven points during a 10-1 run to make it 14-5 midway through the quarter.
Hebard scored six during a 12-4 run that gave the Ducks a 15-point lead with nine minutes left in the first half.
Nicole Kornet made three-pointers on back-to-back possessions to pull UCLA (18-6, 9-4) to within six with 7:53 left, but Bando answered with a three-pointer, a layup and another three-pointer to make it 74-60 with three minutes to left. Oregon converted 10 of 12 free throws from there to seal it.
Jordin Canada had 20 points, six rebounds and seven assists and Billings contributed 18 and 12 rebounds for UCLA, which has lost two of three after winning six in a row.
Sydney Wiese had 17 points to lead Oregon State (22-3, 11-2 Pac-12). With two assists, Wiese moved within one of Leilani Estavan's career record (577) for the Beavers.
The victory snapped a two-game skid for USC (13-11, 4-9). Kristen Simon added 14 points and seven rebounds for the Trojans.
Valerie Higgins' jumper gave USC a 52-40 lead with just more than five minutes left, and Oregon State could not catch up.
The Beavers will play No. 15 UCLA on Sunday. Oregon State's only previous conference loss this season came against the Bruins, 66-56 at Pauley Pavilion on Jan. 13. Earlier on Friday night, Oregon defeated UCLA 84-75.
Portland at Loyola Marymount, 1 p.m.
UC Riverside at UC Davis, 5 p.m.
Pepperdine at San Diego, 6 p.m.
Long Beach State at Cal State Fullerton, 7:30 p.m.
UC Santa Barbara at Cal State Northridge, 7:30 p.m.
UC Irvine at Hawaii, 9:30 p.m.