Ensign O'Neill was a Starfleet officer in the 23rd century. This person served aboard the USS Enterprise in 2267 under the command of Captain James T. Kirk, during the historic five-year mission.
That same year, O'Neill was assigned to landing party number 2, under the command of Lieutenant Kelowitz. This party was searching for the missing shuttlecraft Galileo, which had crashed on the planet Taurus II. When the party was ambushed by the planet's inhabitants, O'Neill was killed by a stone spear through the body. (TOS: "The Galileo Seven")
This character was only mentioned in dialogue.