Lauren Chamberlain drove in the first two runs with a double in the third inning and a single in the fourth. Casey's drive to straightaway center for Oklahoma's NCAA-best 97th home run of the season made it 3-0 in the sixth inning.
Ricketts (35-7) dazzled with a sharp fastball and a biting changeup, keeping the Golden Bears (57-6) off balance throughout the night. She finished two strikeouts shy of U.S. Olympic gold medalist Cat Osterman's World Series record of 18 strikeouts in a seven-inning game in 2006.
"I just had my adrenaline going in this game and I was hitting my spots," Ricketts said.
The fourth-seeded Sooners (52-8) are off until Sunday, needing one more win in two chances to reach the finals. California will face an elimination game Saturday night.
"We just did not play our best ball today. We did not hit the ball well and defensively I don't even remember last time I've seen this team make three errors," Bears coach Diane Ninemire said. "What I do know is that this team is a very strong team, I know that our backs are against the wall but I know that they'll be able to come back from this."
California's best scoring chance came in the sixth, when Jamia Reid started the inning by reaching on a grounder to third when Chamberlain dropped the throw at first base. The Sooners opted to walk Valerie Arioto, a national player of the year finalist, to bring the tying run to the plate. Reid stole third on the final pitch of Arioto's walk, and Arioto swiped second moments later.
Ricketts then struck out Frani Echavarria swinging and Jace Williams looking to end the inning.
"I think she showed why she's the player of the year," Arioto said. "She did a good job of keeping us off balance. Obviously, we need to do a better job of putting the ball in play."
Williams had one of the Bears' hits on a chopper to third with one out in the second. Victoria Jones, who put the ball in play in all three of her at-bats, singled to left with two outs in the seventh.
"She throws the ball hard and she paints the corners. Her changeup was on, it was dropping off the table," Williams said. "She really just came out and really threw a great ballgame."
Ricketts struck out the side in the first, third and fourth innings and tied her season high for strikeouts.
Ricketts is 7-0 in the postseason, allowing only one earned run in 45 innings while striking out 76.
"It's fun watching her approach and how she takes the field because she gets the rest of the team on the same page with her and she lifts us up to a new level," Chamberlain said.
"Especially in the postseason, that's what you need from your team leader."