Shaq -- Goes Bonkers Over Hippy Rapper ... I'm Your Mentor Now! (VIDEO)

Big Shaq got TURNT ALL THE WAY UP at a rap show in Oklahoma City -- getting onstage and headbanging along ... while a jacked muscle dude joined the gang and started flexing on the NBA legend.

Sounds weird, we know ... but it's really funny.

Backstory -- Shaq was hanging out at a bar when rapper Chief Peace took the stage during an open mic night and performed his signature song, "Hippy Kid."

Shaq LOVED it -- joined the guy onstage and started dancing away. Some buff guy joined the gang and took his shirt off for some reason. It's weird. It's also fun.

We spoke with Chief Peace who tells us Shaq spent time with him after the show and gave him all sorts of advice -- and even put his song in rotation on Shaq Fu Radio.