Tons of Oakland Athletics legends have made the trip to Northern California for the team's swan song in the East Bay ... including former ace Barry Zito -- who was actually called upon to sing the national anthem at Thursday's finale!
The pitcher-turned-musician helped conclude the pre-game festivities at the Athletics' last-ever contest inside the Oakland Coliseum ... by belting out a sweet rendition of the Star-Spangled Banner.
The three-time All-Star -- who made the transition to music following his retirement from the sport in 2015 -- picked up the mic just before first pitch against the Rangers .