John O'Callaghan is an Irish musician and DJ, mainly producing trance music.He also produces music under the alias of Joint Operations Centre, Mannix, Henrik Zuberstein and Stenna. He has also collaborated with fellow Discover artists such as Bryan Kearney, Neal Scarborough (as Inertia), Thomas Bronzwaer (as Lost World) and Greg Downey as well as more recently, trance hero Leon Bolier. His best known track is Big Sky, done with vocalist Audrey Gallagher. Callaghan has received remixes by trance artists such as The Thrillseekers, Sean Tyas, Gareth Emery, Cosmic Gate, Markus Schulz and Indecent Noise. He has also worked with Heatbeat and Aly & Fila.