Denis Hurley will provide cover for Ireland this weekend so he is unavailable to the province, while Marcus Horan gets the nod over Wian du Preez at loosehead prop for the visit of the Cardiff Blues.
The absence of Hurley affords Tony McGahan the opportunity to reinstall Felix Jones at full-back. Jones looked sharp when he came on as a second half replacement in Treviso last weekend.
Tommy O'Donnell, who also returned from injury during that game, takes over on the openside of the scrum from O'Mahony.
Ian Keatley, who missed out on the trip to Italy after twisting an ankle, will wear the number 10 jersey in O'Gara's absence and Billy Holland will operate in the second row alongside captain Mick O'Driscoll.
Meanwhile, Munster's long-serving scrum half Peter Stringer will move to Newcastle Falcons on a loan basis once his tenure in Saracens is over.
That will be in early March and it is expected that he will be available for selection for the Falcons for their home game against Harlequins on March 12.
The Ireland international has been on loan from Munster to Saracens since December, providing cover for the Aviva Premiership champions following serious injuries to Neil de Kock and Richard Wigglesworth.
Mark McCall, Saracens' director of rugby, said: "Peter has been a brilliant addition to the squad, on and off the field, and we are sorry his three-month loan spell will come to an end after our match against Northampton on March 4."
Stringer, who turned 34 in December, will remain with Newcastle until the end of the current season before returning to Munster.
MUNSTER (v Cardiff Blues): Felix Jones; Johne Murphy, Danny Barnes, Lifeimi Mafi, Simon Zebo; Ian Keatley, Tomas O'Leary; Marcus Horan, Damien Varley, BJ Botha, Billy Holland, Mick O'Driscoll (capt), David O'Callaghan, Tommy O'Donnell, James Coughlan.
Replacements: Denis Fogarty, Wian du Preez, Stephen Archer, Dave Foley, Paddy Butler, Duncan Williams, Scott Deasy, Luke O'Dea.