Wallingford secured a home play-off for a place in South West 2 after defeating Milton Keynes 28-14 in Southern Counties North.
They will host Southern Counties South runners-up Wootton Bassett on Saturday after the Wiltshire side could only draw 12-12 with Trowbridge.
Wallingford finished level on points with champions Olney, but had a worse points difference.
They took the lead after just three minutes with a penalty from full back Henry Venners.
Wallingford kept the tempo of the game high and increased the lead five minutes later with a well-worked try, finished by wing Nick Risk and converted by Venners.
Next up, flanker Mike Chapman powered his way over from a lineout.
A remarkable spell five minutes before half-time saw Milton bag two converted tries to trail by just one point.
Wallingford re-gathered their composure after the break with some good positional kicking from fly half Henry Pooler.
They finally managed to increase the lead with another Venners penalty.
No 8 Ian Jeffreys broke from the back of a scrum and showed great pace and strength to go over for a try.
Wallingford scored the final points with the last play of the game when lock Mike Turner blasted over from Jeffrey's break.
Witney ensured Amersham & Chiltern's relegation with a 37-11 victory at Hailey Road.
Their five-try win saw them finish a frustrating season in a creditable fifth place.
Witney started well and slick passing saw Will Worrall and Joe Collins release full back Johnny Wheater to score.
Amersham levelled with a well-worked try before their fly half slotted two penalties.
From a turnover ball in the Amersham 22, slick hands put Joe Collins in space to glided over.
Trailing 11-10 at the break, Witney upped their game with the back row of Will Worrall, Carl Campbell and Jimi Collis influential.
The clincher came from Wheater, who showed classy footwork to round his opposite number to score.
Henry Lamb slotted the conversion and then a penalty only minutes later.
Lamb then set up a try for his twin brother James, which Henry converted before adding a penalty.
Wing James Monks shrugged off some would-be tacklers to run in from 30 metres and Henry Lamb converted.
Grove lost 22-16 at home to Swindon in a game they felt was ruined by an over-fussy referee.
Swindon led thanks to a pushover try on ten minutes, but Grove replied with two Roger Sevier penalties.
Grove hooker Steve Dixey and flanker Zach Grant needed hospital treatment for leg injuries before Swindon scored a converted try.
The hosts lost centre Ben Nicholl and prop Richard Keen to fairly dubious yellow cards A Swindon player was lucky to escape the same fate for a high tackle on replacement Simon Cramp, but Sevier kicked the penalty.
Another converted try made it 19-9, but centre Tom Clipsham's interception score and Sevier's conversion closed the gap.
Swindon sealed the points with a penalty.
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