Ensign O'Halloran was the role played by a member of Species 8472, a male Starfleet sciences division officer in the 24th century.

In 2375, the Species 8472 member who impersonated O'Halloran trained at Terrasphere 8 for infiltrating the Federation. This O'Halloran had problems in keeping his Human form and suffered three reverts in one week; had he suffered one more, he would have been kicked out of the program. (VOY: "In the Flesh")

O'Halloran was played by background actor Dan Tirman who received no credit for his appearance.
He filmed his scenes on Monday 13 July 1998 and Tuesday 14 July 1998 on Paramount Stage 16.
It is unclear if there was a real O'Halloran in Starfleet, or if the persona was invented by Species 8472.The Star Trek Encyclopedia (4th ed., vol. 2, p. 96) described the persona created by Species 8472 as a re-creation of Ensign O'Halloran.