Sydney-born Geoff O'Reilly began playing the French horn at the age of 14, having spent a number of years playing tenor horn in brass bands in Sydney, Gosford and Newcastle. In 1993 he graduated from the Canberra School of Music, before returning to Sydney where he played with the Australian Opera and Ballet Orchestra, Australian Chamber Orchestra, Australian Brandenburg Orchestra and the Sydney Symphony Orchestra. During this time he also performed with the Adelaide Symphony Orchestra.
Between 1991 and 1997, he made frequent trips to Germany to study with Professor Ab Koster at the Hamburg Conservatorium of Music. His other teachers have included Reg Bishop, Allan Mann, Clarence Mellor, Campbell Barnes, Dominic Harvey and Hector McDonald.
Geoff O'Reilly was appointed to the horn section of the Sydney Symphony Orchestra in 1995, and in 1998 he took up the post of French horn lecturer at the Newcastle Conservatorium of Music.