During this period, his composition Sainte won the Vocal category of the 2005 British Composer Awards[12] and his debut disc, VOICES was released on the Collegium label.

[13] Beginning in 2007, O'Regan began dividing his time between the UK and the US when he was appointed Fellow Commoner in the Creative Arts at Trinity College, Cambridge, a position he held until 2009.

[8] During his tenure at Cambridge, his composition Threshold of Night won the Liturgical category of the 2007 British Composer Awards[14] and Scattered Rhymes, his first CD on the Harmonia Mundi label, performed by the Orlando Consort and the Estonian Philharmonic Chamber Choir conducted by Paul Hillier, was released in 2008.

O'Regan's second disc on the Harmonia Mundi label, Threshold of Night, appeared in late 2008 and awakened a wider interest in his work, demonstrated by the CD garnering two GRAMMY Award nominations in 2009: Best Classical Album and Best Choral Performance.

[20][21][22] In the same year, he was appointed to the Institute for Advanced Study in Princeton as a Director's Visitor and made his BBC Proms debut with Latent Manifest performed by the Royal Philharmonic Orchestra.

[25] In May 2015, Heart of Darkness received its North American premiere in a production by Opera Parallèle, presented by Z Space in San Francisco, California.

[27] On 30 September 2016 Mata Hari was released in DVD and Blu-ray formats by EuroArts, distributed by Warner Classics; the ballet will be revived for a further run in October, 2017.

[28][29] In February 2017, O'Regan's first album of orchestral music, A Celestial Map of the Sky, performed by The Hallé under the direction of Sir Mark Elder and Jamie Phillips, was released on the NMC label.

[36] From 2011 to 2022, O'Regan composed several pieces influenced by his North African heritage, which included his first collaborations with both the Dutch National Ballet and the Australian Chamber Orchestra, which would eventually culminate in a triptych of orchestral works: Raï (2011), Chaâbi (2012) and Trances (2022).

"[43] O'Regan's music is mostly written in tonal, extended-tonal and modal languages (or a combination of all three), often with complicated rhythmic effects and dense textural variation.