O`Brien joins the Bluebirds from new league rivals Hungerford Town.
The experienced 32-year-old joined the Crusaders in August 2012 and is a skilled player who has Football League experience with Newcastle United, Swindon Town and Yeovil Town.
He played in Newcastle`s Under-17 National Championship-winning team. beating Manchester United 5-2 at Old Trafford.
He made his Newcastle first-team debut in January 2006 in a FA Cup tie and made his first Premier League debut in January 2007 against West Ham United.
O`Brien has since played for Hibernian, Swindon, Hull City, Yeovil and Gateshead.
He has also appeared for Republic of Ireland five times.
Meanwhile, Mark Cooper and Chris Allen have both been made available by Collier on loan for the forthcoming season ahead, and Darren Chitty and Matt Jones have both left the club on a mutual agreement.