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")
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.