And Paul O'Connell's team will do it on the back of completing the season's double over fierce domestic rivals Leinster with a 22-5 Magners League victory at the weekend that stretched their lead at the top of the table to a massive seven-point advantage.
"We have reached the stage of the Heineken Cup where it is all or nothing - if you don't perform in the 80 minutes of a knock-out game you are out of the competition," said McGahan.
"It was always our intention to be competitive in both the Heineken Cup and the domestic league and to have both dreams still alive at this stage of the season is excellent. It was a fantastic result against Leinster in a very attritional match and to win an important game like that going into this huge weekend was just what we needed.
"And, although we have been in this position in the past and it is not a new experience for the players, we do have tremendous respect for the Ospreys - not only their international players but for all of those in a quality squad.
"The Ospreys have got a pretty dangerous and confrontational pack of forwards, they run a good line-out and employ an aggressive defensive system.
"They also have a number of key individuals like Shane Williams, James Hook and Mike Phillips who can make long breaks and really attack and challenge opposing formations from deep."
And while the score stands at 2-0 to Munster in Heineken Cup clashes with the Ospreys - and the Irish province won 25-21 at the Liberty Stadium earlier this season - McGahan insists their will be no complacency from his squad boasting 11 Grand Slam winners.
"Aside of the result on Saturday night the main satisfaction for me was seeing the integration back into the squad of our international players for our first game together for eight to nine weeks. They responded superbly and toughed it out against a very good side.
"However, while every side has some areas in which they would like to improve - and we are no different - we go into the quarter-final conscious that the main thing is to keep our focus on ourselves and what we are trying to do and the rest should follow.
"We do have a few players missing, such as key men like Frankie Sheahan and Rua Tipoki, but we are looking forward to what should be a great and magic day."