Against basement boys Aironi, the Ospreys were solid if unspectacular with flying backs Hanno Dirksen and Richard Fussell doing the damage with a try apiece, while Dan Biggar booted eight points in an 18-11 win.
Nick Williams got Aironi's only try of the night while Naas Olivier kicked two penalties to ensure the hosts finished the season with a losing bonus point.
And while Rees was happy enough with his side's commitment, he is only too aware that the Ospreys must raise their game to take their place in the RaboDirect PRO12 final.
"That was a huge defensive effort and Aironi put us on the back foot during the game," said Rees.
"There are certainly things we will have to work on tactically before next week.
"We will certainly need to control the ball better next week because Munster will bring a huge physical presence to the breakdown. We will also need better field position."
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