We got the "Oppenheimer" star Wednesday outside "Jimmy Kimmel Live!" and our photog asked him about De Niro mistakenly standing up when the winner for Best Supporting Actor was being read. It was a quick little mix-up on Bob's part -- but people definitely noticed.

In case you missed it ... RDJ and RDN were both up for the same award -- Best Performance by an Actor in a Supporting Role in any Motion Picture -- which went to "Robert ... Downey Jr." When De Niro heard "Robert" though, he started to stir in his seat ... almost as if he'd won ... only to be let down when a different last name followed.

RDJ's having fun with it all here, telling us ... "Nothing's better than losing."  Mind you, he was doing this in the middle of signing autographs and shooting the s*** a bit.

He's joking, of course, and the reason we know that is because we asked what he meant -- with Downey Jr. adding ... "I just wanted to give you a good quote." Indeed, it was!

Looks like RDJ's still in good spirits after the big win for his role as Lewis Strauss in "Oppenheimer" ... he didn't just bless us, but he also gave autograph hounds what they wanted too. He also walked away with a big fat peace sign for the whole gang.