From 1985 through 2000, Ontiveros played for the Oakland Athletics (1985–88, 1994–95), Philadelphia Phillies (1989–90), Seattle Mariners (1993) and Boston Red Sox (2000).

He attended the University of Michigan where he gained a Bachelor of Science degree in Physical Education.

In a ten-season career, Ontiveros posted a 34-31 record with 19 saves and a 3.67 ERA in 207 games pitched (73 as a starter).

In 1995, Ontiveros earned a trip to the 1995 Major League Baseball All-Star Game after posting an American League-leading ERA of 2.65 in 1994.

In giving up Jeff Conine's game-winning home run, in the same game, Ontiveros was credited with the loss on behalf of starter Randy Johnson.