The boxing legend and Golden Boy honcho decided to show off his pipes with a rendition of the classic Ritchie Valens hit, "La Bamba" ... and TMZ Sports has the hilarious video!!!
We're told ODLH had some time to kill at the Fantasy Springs Resort Casino after the Cancio and Alvarado fight over the weekend ... so he took the stage in the Twelfth Floor Lounge and told the guitarists to start jammin'.
Look, no one's expecting Oscar to have the voice of an angel ... but the dude put his heart into the damn thing and it looks like he's having the time of his life.
Unclear how the audience reacted ... but we're told Oscar's always welcome back for an encore performance if he wants!