Tries from Nicky Smith, Rhys Webb - replaced at half-time with a head injury - Jeff Hassler, Scott Baldwin and Hanno Dirksen secured a bonus-point, plus 17 points from the boot of Dan Biggar and an additional conversion from Sam Davies ensured the four-time champions got off to a flying start.
Wales internationals Alun Wyn Jones and Justin Tipuric were also missing from the starting XV at the Liberty Stadium, suggesting there is even better to come from Ospreys.
But Rees is keen not to get carried away too early into the season.
"We're not reading an awful lot into it," he said.
"We're pleased that we managed to pick up an early bonus point, but we're under no illusions and we're disappointed.
"We flat-lined and the game died for us for a 20-minute period, when we gave them a couple of cheap routes into the game. They had lively runners and they caused us problems.
"We started to then get individual in defence and gave them encouragement to play, which wasn't our intention.
"That said, we had boys coming off the bench and managed to exert some pressure to finish the game."
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