O’Halloran, 33, has been with Celtic since June 2019 and has made 50 appearances for the club across three seasons.
The former Aston Villa academy graduate has represented his country, making two appearances for the Republic of Ireland.
The appointment comes following the departure of Ian Johnson, who stood down as assistant manager earlier this week.
O'Halloran said: “’I’m happy to step up to the new role to support the gaffer, the team and the club in any way required.
“Whether that is on the pitch or off the pitch, I will always give 100% effort to help us progress.”
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