Second-placed Bicester’s outside hopes of catching Berks, Bucks & Oxon Premier leaders Tadley vanished inside 40 minutes of their top-of-the-table clash.
The home side, who finished 23-3 victors, built up an unassailable half-time ad-vantage which they hung on to – despite being down to 13 men at the end.
Craig Raymond got Tadley on the scoreboard with a penalty after just two minutes, and Jake Cottrell quickly followed with an unconverted try.
Bicester’s 8-0 deficit soon increased to 13-0, this time Tadley winger Gareth Cripps-Jones touching down in the corner.
The contest was effectively over by the break after Ben Anderson went over and Raymond added the extras.
Late in the game, Tadley centre Charles Cox was shown a red card and teammate Jonathan Owen a yellow. This was the cue for Bicester to mount a comeback.
They were rewarded for a much-improved second period display when James Holton sent Rob Arthur in for a touchdown.
But the leaders – who used the wind to its full advantage in the opening half – had the last word when Raymond completed the scoring with a penalty.
BBO Div 4 North: Oxford Harlequins 4th 53 (tries: A Ashby 4, G Dickson 2, R Farahani 2, M Hamed; cons: D Todd 2, J Todd, M Todd), Abingdon 2nd 33.