O'Gara has been granted an early release from his contract with French club Racing 92 to become the backs coach for the Super Rugby champions, replacing Leon MacDonald.
O'Gara, who joined Racing as an assistant coach in July 2013, will begin his new role in New Zealand in the new year.
Statements released by the Crusaders and Racing confirmed O'Gara would be released at the end of December.
The 40-year-old's contract had been due to run until 2019.
