Munster will now take on regular season table-toppers Glasgow Warriors in the Irish province's first play-off final since 2011.
But O'Mahony believes a dramatic improvement will be needed if his side are to be crowned champions on Saturday.
"It's great to be involved on a Cup Final week," he told the club's official website. "It has been a while for Munster and these are the weeks a club like us wants to be involved in.
"We played some good rugby (against Ospreys), especially in the first half where we were quite clinical and early in the second half it was good to get Denis' (Hurley) try.
"But unfortunately from there on mistakes started to happen and we made life harder than it should have been for ourselves.
"That's the way these semi-finals can go. Ospreys are a quality side and we were probably guilty of backing off a little bit when we had that cushion on the scoreboard - that was silly of us.
"Still though, we've managed to get ourselves to a final and have the chance to win a trophy so we'll need to learn from our errors against Ospreys."
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