Oscar De La Hoya -- Comeback Is 'Terrible Mistake' ... Says Bob Arum

Oscar De La Hoya would be a complete FOOL to come out of retirement and fight again at 42 ... so says legendary boxing promoter Bob Arum who's telling Oscar -- DON'T COME BACK!!

De La Hoya has been flirting with the idea of getting back in the ring and fighting Gennady Golovkin ... but Arum says it would be a "terrible, terrible mistake."

"He had a great career ... when it's over, it's over."

Arum says Oscar should learn from guys like Sugar Ray Leonard ... who tried to come back at 40 years old ... and looked like crap.

"I don't think Oscar is fool enough to do that."