Oscar De La Hoya says the only way Conor McGregor can beat Floyd Mayweather is with a flying kick to the head ... 'cause there's absolutely ZERO chance he'll out-box the Money Man. 

ODLH says he "wishes" McGregor was on Mayweather's level, but he's simply not -- and tells TMZ Sports if he couldn't beat Floyd during his own boxing heyday, McGregor has no real shot. 

That said, Oscar does dish out some advice for Conor on how to fight Floyd ... but he doubts it's gonna help. 

FYI, Floyd and Oscar fought back in 2007 -- Mayweather won by split decision.