Liberty Ross turned 39 this weekend -- but by how many celebs came out to celebrate with her ... you woulda thought it was one of the big X-0s.

Tons of Hollywood A-listers flocked to Liberty and her husband Jimmy Iovine's house Saturday night in Holmby Hills, where there was certainly no shortage of star power. At least 150 people showed up, including Oprah, Gayle King, Mona ScottDavid Geffen, Courteney Cox, James Corden, Usher, Diddy and Trey Songz ... just to name a few.

Oh, and Missy Elliott performed too ... playing a lot of her hits to a turnt up crowd.

Talk about putting a little English in Hollywood.