Sean Morgan appeared in four Original Series episodes as junior Enterprise officers, most notably Lieutenant O'Neil in The Return of the Archons and The Tholian Web.
Sean Morgan played small parts in all three seasons of The Original Series.
He is first seen as Brenner, a crewman in the phaser control room, in Balance of Terror. In The Return of the Archons he is Lieutenant O'Neil, who is with Sulu on Beta III in the opening scene; Sulu is absorbed, and O'Neil is later found among Landru's placid followers until the computer is destroyed.
In The Ultimate Computer he plays Ensign Harper, the engineer vaporised when the M-5 unit draws power directly from the warp engines. He returned as O'Neil in The Tholian Web, where he is one of the party in environment suits aboard the drifting USS Defiant.