A spirited comeback inspired by two of their younger players earned Horspath an unlikely victory on their first appearance in the top flight in Division 1.
Chasing Banbury Twenty's 145 all out, the hosts were in deep trouble on 74-5.
Paceman Faisal Khalid took three wickets and Keith Arnold bowled a typically tight 11-over spell that yielded figures of 1-17.
But Aussie Ollie Robinson (36no) and Will Eason (32no) saw Horspath to maximum points with 20 overs to spare.
Earlier Mark Haupt (77) had been the only visiting batsman to cope with the pace and swing of Lee Mason, who took 4-55.
Horspath 25pts, Banbury Twenty 7
Dave Powles hit 117 for Great & Little Tew to be the only Division 1 centurion on the opening weekend, but his day ended in disappointment as Cumnor powered to a six-wicket win.
Put in to bat, Tew posted 207-6 from their 52 overs, with medium pacers Ali Morteza (3-46) and Dan Gaunt (3-55) sharing the wickets.
Opener Morteza then hit a swashbuckling 60 to get the visitors off to a flyer, and Andrew Robertson (37), Jamie Douglas (42) and Dan Gaunt (34) set up victory with four overs in hand.
Great & Little Tew 8, Cumnor 25
Alex Jewell and Chris Watling shared an unbeaten 150-run partnership as Aston Rowant enjoyed an easy win in a one-sided affair at Shipton-under-Wychwood.
Rowant, who were relegated from the Home Counties Premier League last term, dismissed Shipton for 154. Ryan Dutton took 3-34, while Sahan Wijeratne top-scored with 55.
The visitors suffered an early setback before Jewell (91no) and Watling (63no) eased them to 158-1.
Shipton-under-Wychwood 3, Aston Rowant 25
Oxford Downs' clash with Oxford 2nd followed an almost identical pattern.
Richard West produced the best bowling figures in the division, taking 6-67 for title favourites Downs, as Oxford were bowled out for 162.
Only Tim Marcon (41) got going after the visitors were asked to bat first.
An opening partnership of 137 between Chris Sandbach (73no) and Frankie Crouch (64) then paved the way for Downs' nine-wicket win.
Oxford Downs 25, Oxford 2nd 3
Last year's runners-up Banbury 2nd returned to winning ways, but were made to work hard for their four-wicket success at home to Bicester & North Oxford.
A battling 82-run partnership between Gary Sansome (70) and Andrew House (43) enabled Bicester to reach 204-6 from their 52 overs after they were put in to bat.
But Lloyd Sabin (57) and Chris Board (75) fared even better, adding 141 as Banbury secured victory in the 50th over of their reply.
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