Munster are just two points behind Philippe Saint-Andre's side with two rounds of the Pool stage remaining and will do battle in the South of France this weekend.
The double Heineken Cup Champions will then be looking to avenge their opening weekend defeat by London Irish when the English side arrive at Thomond Park the following weekend.
"Europe is what it has always been about," said O'Callaghan who has amassed 74 appearances in Europe's premier competition.
"It's a massive week, a cup final week, and we're playing for our lives. We knew when this pool was made that it would come down to one or two games, and this is it, two games to go and it's all on the line.
"This is where you test yourself. This is where you want to be."