Forget Tom Cruise ... Steve-O says HE should be the one to fight Justin Bieber -- telling TMZ Sports that Justin is "picking a fight with the wrong guy!"
We broke the story ... Dana White is hoping to put together a UFC match between the Biebs and Cruise after Justin called out the actor on Twitter.
But, Steve-O says he's personally offended that he wasn't the guy in the initial challenge ... telling us out at LAX, "I'm sick of being overlooked for my potential as a cage fighter!"
Steve-O says he was so upset about the Bieber snub ... he actually flew to Las Vegas to have a personal convo with Dana White himself about it all!!
"I had a little chat with Dana White," Steve-O says ... "And he agreed that if Tom Cruise doesn't step up, and someone needs to fill the cage with Justin Bieber, then I'm the guy."
"We shook on it, me and Dana White."
Steve-O then issued a call-out of his own to Bieber ... telling the Biebs, "I think you've been picking a fight with the wrong guy. Tom Cruise might be 56, but, hey, I'm 45!"
As for who would win in the Octagon ... all Steve-O would tell us is that Justin has NO chance of ending the fight early -- saying, "I can take a serious beating."
He ain't lyin'!!!