TOS Starfleet is a mini playable faction of Star Trek Online.
Starfleet in the 2270s is one where the Federation was a lot smaller, and unexplored space a lot bigger. Relations with both the Klingons and Romulans were tense and captains were a more ready to bend the rules and pull their phasers. This faction is a subset of the main Starfleet faction, but is tied to the aesthetics of Star Trek: The Original Series.