Cahill began his career with Clonmel Town, one of Ireland's top junior football clubs winning an
FAI Junior Cup winners medal with them in 1994. He then spent two seasons at Northampton before returning home in the summer of 1996. Despite playing a friendly for
Waterford United against Southampton he signed for
Cork City. After six seasons at Cork he moved to
Shelbourne in the summer of 2002. While he was with Shelbourne he won the
League of Ireland championship in 2003, 2004 and 2006. Cahill was signed by Drogheda United on 23 December 2006. With Drogheda, Cahill won his fourth Premier Division title in five seasons and a
Setanta Sports Cup winners medal in 2007. Cahill signed for Shamrock Rovers on 16 January 2009 along with
Stephen Bradley in what was
Michael O'Neill's first signings. He departed
Shamrock Rovers in July 2010 after making 53 appearances for the club.and soon signed for
Sporting Fingal. When Sporting went out of business in February 2011, Ollie moved to
Dalymount Park where he joined
Bohemians, making his competitive debut against
Portadown in the
Setanta Sports Cup.[https://web.archive.org/web/20110721124419/http://extratime.ie/newsdesk/articles/5031/ ==European competition==