Three missed penalty kicks by stand-in skipper O'Connor also proved crucial and put a dampener on his 50th Super Rugby appearance.
The Reds were quick to put the knife into the Rebels' second-half hopes with halfback Nick Frisby, in his first start in the No.9 jersey, scoring after some good lead-up work by centre Anthony Faingaa, in his first game this season after a hand injury.
Quade Cooper converted from the sideline and then a penalty kick at the 60-minute mark pushed the lead out to 20-10.
With a couple of breaks out wide and some fierce tackling, Rebels No.8 Scott Higginbotham did his best to rally the team in his first outing against his former side.
But the Rebels lost momentum following the Lance no-try.
But they delivered an impressive first half, with Mitchell's try well deserved.
Electing for another scrum five metres from the tryline rather than a penalty shot, the Rebels attacked the line before halfback Nick Phipps found the hard-working O'Connor.
He held the ball up long enough for Mitchell to run the perfect line, scooting between defenders to touch down.
Queensland's first-half try was scored by centre Ben Tapaui.
A penalty kick by Cooper with just over a minute remaining compounded the misery for the Rebels, who missed out on a bonus point for finishing within seven points.
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-AAP