Ryan Gosling Performs 'I'm Just Ken' with Help of Slash at Oscars

Ryan Gosling did his big musical number at the Oscars  -- getting onstage in front of all his peers and belting out the 'Barbie' standout song ... and he did it guns (n' roses) blazing.

The actor just got done performing 'I'm Just Ken' during Sunday's broadcast -- and it was as epic as everyone expected ... with the Ry Guy fully committing to the bit ... and rocking the house with some killer vocals and even better backing musicians, including Slash!

Indeed ... the top hat-rockin' rocker joined Ryan onstage to play some live guitar, and he absolutely shredded as RG belted out the fan fave from the biggest blockbuster last year.

Ryan even came down to the front row -- where he had Margot Robbie and Greta Gerwig help him sing the tune. And yes, he was fully decked out in all-pink wardrobe for the ditty.

For a little while, it was unclear whether Ryan was actually gonna do 'IJK' at the Oscars -- but in the end ... he finally signed on, and his costar Margot just recently reassured everyone he was itching to do it ... despite him kinda playing coy throughout this year's award season.

It was one of the only 'Barbie' highlights of the night thus far -- Billie Eilish just won Best Song, for 'What Was I Made For,' but it's been fairly quiet for the doll crew otherwise.

We'll see if 'Barbie' ends up getting some love for Best Picture ... or if 1 Oscar is it.