Oklahoma (51-8) next plays Friday night against the winner of Thursday's LSU-California game. The Bulls (50-13) play the loser of LSU-California on Saturday in the first elimination game.
Ricketts, the USA Softball Collegiate Player of the Year, struck out 10 or more for the 23rd time this season.
South Florida, playing in the WCWS for the first time, took a 1-0 lead in the top of the fourth inning on an RBI double by Kourtney Salvarola.
Oklahoma answered with Chamberlain's two-run homer in the bottom half of the inning. Ricketts' triple highlighted Oklahoma's three-run sixth.