Grove were suprisingly beaten 39-15 at struggling Newbury Stags in Southern Counties North.
The visitors felt that only Chinnor, in the Oxfordshire Cup, had proved tougher opponents this season.
Stags had the game effectively won within the first ten minutes by running in three spectacular tries, one converted, to put them 17-0 up.
Grove hit back when fly half Nick Sevier exchanged passes with centre Dave Johnson to score, while Roger Sevier converted.
Roger Sevier added a penalty, but a converted try made it 24-10 at half-time.
Nick Sevier's solo try was Grove's only other score as Stags bagged 15 more points to leave the visitors wondering which Newbury team they had played against.
Grove's woe included replacement hooker Steve Dixey being sin-binned just 30 seconds after coming on.
Harwell improved their promotion chances in BB&O Division 1 South by winning 24-15 at Oxford Harlequins 3rd.
The visitors led 12 -3 at half-time thanks to a brace of tries from winger Ryan Hodnett, and Dave Beaven's conversion.
A try double from Harwell captain Andy Lee, plus a Beaven conversion, sealed the result.
Full back Darrel Rendall's injury-time penalty gave Littlemore a 17-15 victory at home to Hungerford in BB&O Division 2 South.
Playing in front of 40 former players, Littlemore led 14-3 at the break with tries from flanker Darren Massey and Rendall, who converted both scores.
Hungerford hit back to lead 15-14 before Rendall's dramatic winner.
OTHER SCORERS
BB&O Premier A: Windsor 2nd 23, Witney 2nd 18 (tries N Phillips, A Lambourne; con S Copperwheat; pens Copperwheat 2), Henley Wanderers 2nd 25, Oxford Harlequins 2nd 31 (tries N Spencer, M Riley, D Lynchsmith, J Keown, C Mawle; cons Riley 3).
BB&O Division 1 South: Oxford Harlequins 3rd 15 (tries J Clapp, M Jones; con: T Young; pen Young, Harwell 24.
BB&O Division 2 North: Stow-the-Wold 3rd 0, Bicester 2nd 39 (tries A Smith 2, A Field 2, G Williams, T Edgar, A Blacklock; cons Smith 2).
Friendly: Chipping Norton 2nd 44, Oxford Harlequins 4th 7 (tries R Telling; con Telling).
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