Cumnor will take on Horspath in the Oxfordshire final after both secured narrow victories.
Cumnor beat Great & Little Tew by six wickets, but with just two balls remaining, while Horspath saw off Aston Rowant by three wickets, despite being 44-6.
Dean Creed was Horspath's match-winner, taking 6-23 from his nine overs, five of his victims being bowled.
Rowant opted to bat first, but Chris Watling was the only batsman to handle the Horspath bowling as wickets fell at regular intervals.
They eventually limped up to 122-9 off their 40 overs, helped by an unlikely last-wicket partnership of 28 from Ryan Dutton and Chris Davis.
This gave Rowant a much-needed lift and they produced a devastating start to the Horspath innings.
Taking six quick wickets, Horspath were deep in the mire at 44-6, with only No 3 Robbie Eason holding firm.
A 33-run partnership with Creed put the match in the balance, before Dave Heritage came to the crease to help Eason steer Horspath home - helped by 44 extras.
At Great Tew, the hosts' went for their shots from the off, but lost wickets at regular intervals.
Steve Murrey was the first man to go, caught at long on, before Shane Faulkner was unluckily run out.
His firm shot was deflected onto the square leg umpire by a fielder, who then rifled the ball in to run Faulkner out.
County under 17 captain Jack Taylor hit three sixes off Ali Morteza, as he helped Tew up to 151-9.
Good opening spells from Lloyd Brock and Kevin Taylor put Cumnor on the back foot at 43-2 after 20 overs.
But a well-constructed 39 from Dan Gaunt, supported by Morteza and Jamie Douglas, got the visitors back into the game.
Despite a hiccup of two run outs, a cameo of 16 from seven balls from Mark Kirsten saw Cumnor home in the last over with two balls to spare.
The final is at Horspath on Sunday, June 15.
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