Layshia Clarendon scored 14 points and No. 6 California won its record-tying 12th straight with a 77-55 victory over the Ducks.
Oregon (4-23, 2-14 Pac-12) entered the game overmatched on paper, yet the Ducks overcame a 10-0 deficit and actually held a one-point lead with just under four minutes left in the first half.
But Cal (24-2, 14-1) started pulling away with a 12-2 run to close the first half en route to matching the record win streak set by the 2008-09 team.
"I showed up here tonight to win the game," Westhead said. "We didn't win, so I have no encouragement."
Jordan Loera and Ariel Thomas scored nine points each to lead the Ducks (4-23, 2-13). Leading scorer Jillian Alleyne was held to just seven.
Afure Jemerigbe scored 13 points for the Bears, who early on looked somewhat ragged offensively with starting point guard Brittany Boyd sitting out because of a strained groin.
Jemerigbe came in averaging just 7.0 points, so her offensive contribution was big with Boyd out of the lineup.
Westhead was most impressed with Cal's inside muscle.
"They're a relentless offensive rebounding team. They might be as good as there is in the country," he said.
Boyd, a sophomore who leads Cal in assists and ranks third in scoring at 12.5 points per game, sat among her teammates throughout the first half. She came out in the second half with her hair tied up and was sitting next to the coaches when play resumed, as though she'd be ready if needed.
The Bears turned a nine-point halftime lead into an 18-point cushion midway through the second half and remained tied with Stanford atop the conference standings. If Cal wins its final three games, it will gain at least a share of its first ever Pac-12 title in women's basketball.
After shooting just 29 percent in the first half, the Bears improved to 55 percent (17 for 31) in the second half.
The Ducks shot just 25.4 percent (15 for 59) for the game.
"I thought we were still involved in the second half," Westhead said. "Cal played very well. At the end of the day, you've gotta make shots. You can't shoot 25 percent."
The Ducks' poor shooting included a 6-for-26 showing from beyond the 3-point line.
Boyd's absence was apparent in the first half, as the Bears hit just 8 of their first 33 shots and looked disjointed offensively. Missing several good looks from the paint didn't help their cause.
Cal shot 29 percent (12 for 41) overall in the first half, but the Ducks were even worse at 23 percent (7 for 31).
Oregon, which trailed 10-0 and didn't score until 5 minutes in, took a 19-18 lead on Devyn Galland's free throw with 3:49 left in the first half. But Cal's closing run, highlighted by two 3-pointers from Mikayla Lyles and five points from Clarendon, gave the Bears a 30-21 lead at intermission.