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Before Oscar De La Hoya hit Donald Trump with hardcore cheating allegations -- he was singing Trump's praises as a "great golfer" after personally watching the guy nail an impressive shot.

Both Oscar -- and George Lopez -- gave video testimony about Trump's incredible golf skills following an outing with the businessman in April 2010 at Trump National Golf Club Los Angeles.

In the video, obtained by TMZ Sports, Trump walks up to the group, takes a club and crushes a tee shot at the 15th hole ... putting the ball feet from the pin on a par 3 without even warming up.

"I'm actually very surprised that he hit his tee shot so close to the pin because that was his first shot of the day," Oscar said ... "What does that tell you? He's a great golfer."

George Lopez also went on camera and praised Trump for the shot.

It's a COMPLETELY different story from the one De La Hoya told this week -- in which he painted Trump as a bumbling cheater who repeatedly tried to pull fast ones on his fellow players.

But we're told Trump didn't even play with Oscar that day -- he only stepped on the course for that ONE shot and left. He claims it's the only time he's EVER been on a golf course with the boxer.