Katy Perry -- First Big Night Out Since Split with Russell Brand

Katy Perry made her first big public appearance in the U.S. since her split with Russell Brand last night ... posing on the red carpet with her BFF, jewelry designer Markus Molinari, at the grand opening of 1 OAK Las Vegas at the Mirage Hotel & Casino.

Even though Katy was on the flyer for the event, our guy inside says Katy never addressed the crowd and stayed in her booth (behind a wall of security) for the entire night.