Harwell secured promotion to the BB&O Premier Division after thrashing Aldermaston 78-0 in Division 1 South.
They will replace Wheatley in the Premier after finishing as the highest-ranked first team in 1 South and North.
Flanker Kevin Wood starred with an incredible six-try haul, while Andy Lee (3), Dave Beaven, Dave Paton and Ryan Hodnett also crossed over.
Beaven finished with nine conversions.
Also going up are Wallingford 2nd, who topped Division 1 South after beating Slough 63-0.
Hooker Liam Wordley scored Henley Hawks' only try as they lost 22-5 at home to Redruth in National 2.
Their final game is at Cambridge on Saturday.
OTHER SCORERS
BB&O Premier A: Witney 2nd 26, Oxford Harlequins 24 (tries N Spencer, C Mawle, C Upham, K MacLennan; cons M Riley 2).
BB&O Division 1 South: Oxford Harlequins 3rd 55 (tries P Gray 3, I Phillips, P Moss, M Stoddart, M Jones, J Clapp, T Mardle; cons T Young 3, Jones, I Phillips), Grove 2nd 20 (tries B Law, J North; con R Martin; pen Martin).
BB&O Division 2 South: Grove 3rd 17 (tries C Boshell 2, G Nicholl; con M Blow), Redingensians 3rd 37.
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