Ensign O'Neill was a 23rd century Starfleet officer serving aboard the USS Enterprise under Captain James T. Kirk during the mid-2260s.

In 2266, O'Neill was a member of the landing party searching for the crashed shuttlecraft Galileo on the planet Taurus II. O'Neill was among the crew killed by the savage anthropoids that inhabited the planet. (TOS: "The Galileo Seven")