From Halopedia, the Halo wiki
O'Brien entered the SPARTAN-IV program as a recruit, where they were put through training at the Avery J. Johnson Academy of Military Science. O'Brien was later seen standing alone while Spartans Ionescu, Denning and Page were socializing nearby. O'Brien took part in a live-fire exercise in which they competed alongside Spartans Leung, Eusebio, and Denning. They faced off against Spartan Commander Laurette Agryna, Spartans Ionescu and Page, and one other spartan recruit.
List of appearances