Oscar De La Hoya -- TRUMP'S A GOLF CHEATER ... I Saw It Myself

Donald Trump "outright cheated" during a golf outing with Oscar De La Hoya ... so says the boxer himself who claims Trump resorted to dirty tricks just to beat him on the links.

Oscar was on "The Score" radio show in Vegas when he recanted his golf match with Trump at Donald's own golf course in Los Angeles.

"I witnessed it. He outright cheated," Oscar said ... "not once, not twice but 3 times."

You gotta listen to Oscar tell the story ... the Golden Boy says he was cracking up the entire time because "you have a man who's a billionaire and he has to cheat just to say he beat me."