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Oscar De La Hoya isn't backing down from Dana White -- and doubled down on bashing the Floyd vs. Conor fight ... even after White tore him a new one.

Oscar appeared on "SportsCenter" on ESPN and directly responded to the expletive-filled tongue lashing White delivered to the boxing legend on "TMZ Sports."

White had said Oscar's criticism stemmed from his own insecurity because ODLH's last fight -- Canelo vs. Chavez -- "sucked."

Well, Oscar says Floyd vs. Conor will suck even worse ... and explained why.

Oscar says he still has respect for Dana ... but we doubt these two are hanging out any time soon.