Cahill had a youth career with Clonmel Town. He then moved to English outfit Northampton Town. He later returned to Ireland for spells with Cork City, Shelbourne, Drogheda United, Shamrock Rovers, Sporting Fingal and Bohemians.
Cahill began his career with Clonmel Town Football Club, 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. Cahill was signed by Drogheda United on 23 December 2006.
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. Cahill jointly holds the record with Owen Heary for most appearances in for an Irish domestic player (41).
He scored for Shelbourne against Olimpija Ljubljana in August 2003 and again for Drogheda United against Levadia Tallinn on 16 July 2008 in a Union of European Football Associations Champions League qualifier.
Cahill retired from the game at the end of the 2011 season.