Oakland A's Rookies -- HIT THE CLUB ... In Sexy Dresses (VIDEO)

The Oakland A's definitely put the "A" in T&A Sunday night ... when they put their rookies in a bunch of sexy dresses and made them hit the L.A. club scene -- and TMZ Sports has the footage.

A bunch of rooks including Max Muncy (Supergirl), RJ Alvarez (french maid), Pat Venditte (Reno 911) and more hit up Greystone Manor nightclub in their rookie hazing costumes ... well, because it was funny.

Muncy even tweeted about it ... saying, "Sometimes when you're a rookie you have to dress up."

True.