The Welsh region finished this year's campaign with a convincing 45-20 defeat of Connacht as they ran in six tries to become the top try and point scorers in the RaboDirect PRO12 this season.
Ashley Beck bagged two tries to go with scores from Alun-Wyn Jones, Dan Baker, Nicky Smith and Jeff Hassler for the Ospreys, who saw their play-off hopes dashed last weekend with defeat to Zebre.
And despite ending their season with a win, Rees insists the overall disappointment the entire squad feels will only serve as motivation to improve for next year.
"There's definitely some lingering regrets. I can't deny that" said Rees. "When you look at not just last week, but different parts of the season, times when players are away, certain games where we've lost in certain
periods, they do catch up in the end.
"We wanted to add some try scoring potency to the team this year and we've done that. We've been Rabo top try scorers, and I know there's been a bit of 'flat-track bullying at times', but my view is everyone plays everyone so I am pleased with that aspect.
"66 points has got sides consistently into the semi-final in previous years, we've got in there with 62 before now. Ultimately though, we haven't nailed the top four.
"We've had a good, professional performance against Connacht but there's some disappointment that hopefully will carry though to next season and make us a more dangerous and resentful animal next year.
"We need to up the ante in terms of how we prepare, to enable ourselves to be able to win on a weekly basis, to make sure we are the best we can be every week, because we haven't always done that this season."
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