Ronan O'Gara converted both and kicked four penalties to complete Munster's scoring while Castres responded with a try from Yannick Caballero and a conversion and three points from Pierre Bernard.
Munster will now finish the group at Northampton looking to secure a home quarter-final, while the Saints are aiming for victory to progress to at least the Amlin Challenge Cup equivalent.
When the two sides met in November O'Gara sunk a drop goal with the last kick of the game to seal a 23-21 win for Munster - who have won all five of their Heineken Cup games so far.
And with that triumph still fresh in the mind O'Connell, named as Ireland's captain for the upcoming RBS 6 Nations last month, is expecting a game the scale of a Test match.
"Northampton still have a lot to play for. There is a bit of history between us and we have played each other a lot over the last few seasons," said O'Connell.
"It will be almost like a Test match. We know we'll have a home quarter-final if we can get a result over there and that would be brilliant."
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