Munster have been in impressive form so far this season in the GUINNESS PRO12, winning seven of their nine games to sit top of the table.
But the 16-9 defeat to Clermont over the weekend leaves their hopes of progressing in Europe in the balance.
Clermont are the first French side ever to win at Munster thanks to scores from Fritz Lee and dazzling centre Wesley Fofana, added to six points from Camille Lopez's boot.
Munster only managed three penalties from Ian Keatley, and captain O'Mahony admitted that his forwards need to step up come Saturday's return fixture in France.
"It's fairly simple, we have got to dominate the physicality - I think we were bullied at times which is not something that normally happens at home but we have got eight days to recover," he said.
"We came up against an excellent side, we knew it was always going to be tough.
"I though they played very well. I thought we could have done a lot of the things we did better.
"It's half-time, that's the beauty of this competition and these back-to-backs and we've just got to put it right next week.
"We have to put our best foot forward and place ourselves in the best possible position to get a result on Saturday.
"We let ourselves and the people here down- we have got to get it right next week, that's the bottom line."
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