De La Hoya knocked Ruelas down twice before the fight was stopped in the second round.The IBF then ordered De La Hoya to defend against Miguel Julio.The WBC awarded De La Hoya their welterweight title, which he lost, to Shane Mosley (34–0) by a split decision on 17 June 2000, giving De La Hoya the first sound defeat of his pro career.The fight was a disputed decision, with one judge scoring the fight 115–113 for De La Hoya, and the other two scoring it 116–112 and 115–113 for Mosley.After seven defenses of his lineal and WBC welterweight titles, De La Hoya fought rival and IBF Champion Félix Trinidad (35–0) on September 18, 1999, in one of the biggest pay-per-view events in history, setting a record for a non-heavyweight fight.Oscar dominated the vast majority of the first nine rounds, staying just outside Trinidad's range while generating much success with his stiff jab and blitzing combinations.De La Hoya took promoter Bob Arum to court in the fall of 2000, trying to break his contract with the promoter.The courts ruled in favor of De La Hoya in February 2001.
On May 6, 1995, De La Hoya defeated IBF lightweight champion Rafael Ruelas (43–1–0) in a unification bout.
He relinquished the IBF title and defended the WBO title against undefeated Genaro Hernández (32–0–1), who relinquished the WBA super-featherweight title to fight De La Hoya.
Hernandez quit after six rounds because of a broken nose. The fight was stopped due to a bad cut suffered by Chavez.
In his sixth and final defense of the WBO lightweight title, he knocked out Jesse James Leija (30–1–2) in two rounds at New York's Madison Square Garden. Until their rematch in 1998, Chávez stated that De La Hoya did not defeat him since the fight was stopped.
De La Hoya successfully defended his titles with a twelve-round unanimous decision against undefeated former WBC Lightweight Champion and number one light welterweight contender Miguel Ángel González (41–0–0).
He defended the title once, stopping Giorgio Campanella (20–0) in three rounds.