Overlook “That ’70s Present” — Laura Prepon had herself a karaoke present in Nashville over the weekend … fearlessly hopping up on stage and belting out Weezer’s “Say It Ain’t So.”

Test it out … the actress grabbed the mic at The Lipstick Lounge and seemed like she was having a blast as she labored the stage … encouraging the viewers to sing alongside and sharing her mic with the gang. She gave the impression to be within the firm of some pals, too.

An eyewitness named Sav tells TMZ the bell-bottomed magnificence appeared “so all the way down to earth” and “identical to a daily woman hanging along with her pals” as she commanded the stage. Sav provides Laura had incredible vitality and was “interacting with everybody.”

Sav even admits she did not understand Laura was truly Alex Voss from “Orange Is the New Black” — which Laura starred on for 7 seasons starting in 2013. Our eyewitness says she even instructed Laura she seems to be just like the spitting picture of her ‘OITNB’ character … and that she playfully “smirked” in response.

Sav gushes it was “such a cool second to have the ability to catch” and the gang liked Laura’s “iconic” track selection.