Oakland sent lefty Eric O'Flaherty and cash to the Mets on Tuesday.
Frias, 23, has pitched at two levels of Class-A ball.
He started the year at extended spring training before going to Single-A Savannah on May 5. He went 0-2 with an 8.71 ERA and a save in 15 relief appearances then was transferred to Single-A Brooklyn on July 19. There, he had a 3.38 ERA in four relief outings.