Credit: Araya Diaz/

When Angie and the kids are away (in England), Brad will play!

Brad Pitt attended Monday's Oakland, Calif. premiere of his buzzed-about baseball epic Moneyball -- and, after charming reporters on the red carpet, he briefly hit the afterparty. 

Sans love Angelina Jolie or their six kids (who reportedly stayed behind in London), Pitt let "loose" at the bash, according to a witness.

"He was hugging old ladies, taking pics with costars, and letting the crowd photograph him with cell cameras," the observer tells Us Weekly, adding that the 47-year-old star had thrown off his jacket and unbuttoned his shirt.

As he drank from a champagne flute, the hunky star "even danced for a second to the 80s music blaring!"

"He was charming the pants off people," adds the witness of the star, who is gaining Oscar buzz for his performance in the flick. "Smiling, laughing, backslapping, very touchy feely with people...and chewed gum through it all."

PHOTOS: See more pics of Brad letting loose from PopSugar

Before the bash, Pitt gushed on the red carpet about making the film, based on author Michael Lewis' fact-based book of the same name, about the Oakland A's brilliant manager Billy Beane, whom he had the opportunity to hang with as the film languished in development.

"We mainly sat around and had a laugh. He is one funny guy," he said of Beane. "He and everyone with the A's were really gracious, opening doors for us."