1) John O'Callaghan is an Irish DJ, mainly producing electronic music. He also produces music under the alias of Joint Operations Centre, Mannix 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. His best known track is Big Sky, done with vocalist Audrey Gallagher, formerly of the 90's alt-rock band Scheer.

O'Callaghan has had the majority of his work released on Discover, the British record label set up by John Askew. Recently, his work has been released by sub-labels of the Armada Music label; primarily on Armind and Soundpiercing. The remix of his vocal trance production "Big Sky" by Agnelli & Nelson was voted “Tune of The Year” by listeners of Armin van Buuren's A State of Trance radio show in 2007.

In addition to numerous single releases, he has released a live album (Discover "Live As" Volume 2) and an artist album, Something To Live For. He entered the DJ Mag 100 in 2008 at number 60.

In 2009 he released his second artist album "Never Fade Away", which includes famous track "Big Sky".

2) John O'Callaghan is the singer of the Arizona American based pop group, 'The Maine'.