"It was absolutely vital for us to win this one - lose it and we were out of Europe, simple as that," said Heineken man-of-the-match O'Gara after last Saturday's 29-16 victory over the Scarlets at Stradey Park.
"There are going to be plenty of sub plots going on but the calibre of the teams in this pool is incredible and you have to fight for every point, be it even a losing bonus point.
"Llanelli Scarlets are a quality side and you have to be very careful not to write them off because, with the type of rugby they play, they can attack from anywhere and be dangerous. This time last year they were going about winning seven on the bounce and they have not become a poor side overnight.
"We all saw how dangerous they can be and that is an area we have to look at before Sunday and, although it is easier to win at home, the boys really fronted up at Stradey Park and that kick-started our campaign. Now we have to move on from here.
"It was a big ask to front up and I thought the pack was great. Last week we weren't mentally in the right place to face Leinster and it was important we came back in style," added the Ireland number 10.
"The bit was between the teeth, the blood was up and the pack was magnificent, particularly some of them who were under pressure. They answered back and I always knew the quality was there."
Meanwhile, Munster coach Declan Kidney also accepts that the Scarlets job is only half done.
"The Scarlets are not gone, they are still going to have an effect on this group. Next weekend is the second of our back-to-back fixtures and, while it was a huge achievement to come over here and win, very few sides get the better of them two weekends running," Kidney explained.
â00We are top of the group but the time to be top is after Round 6. We have been there or thereabouts before at this stage and know it is of no consequence other than it makes it easier to go training on Monday and Tuesday.
"We won't object to being top but we have a home game coming up and it is hugely important that we remember the Scarlets are a very good side and that we will need all our experience for that game.
"It is going to need every player at his best because they will come out looking for their pound of flesh and that is what makes them such a great team - and why there is such great and healthy respect between the two sides," he said.
"We were in a bit of trouble in the first half but we regained our composure and managed to get in front and score a try from a bit of quick thinking. That meant we didn't have to force the game and just play the game down.
"The conditions were horrendous and it was extremely difficult for all 31 out there on the pitch - nobody knew that more than the referee - but the pack played well when it really was a night for the forwards."