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Volts off-spinner Mark Craig sliced a thick edge down to the vacant third-man boundary to seal the win from the last ball of the match.
Otago has now won nine games in a row and is one win away from what would be almost the perfect season.
The Volts have lost just one twenty/20 match this summer and will host the final at the Oval on Sunday against either Wellington or Auckland.
Auckland beat Central Districts by six wickets with a little help from the Duckworth-Lewis method and will play Wellington in the capital on Friday for the right to join the Volts in the final.
Craig's streaky boundary was welcomed with thunderous applause from a crowd of just over 1500, who had turned out for the midweek game played in gloomy conditions.
No doubt a few extra spectators were lured by the opportunity to see former Australian fast bowler Brett Lee make his Otago debut.
The paceman was flown in for one match and did not disappoint. He bowled with plenty of heat and took a wicket with his final ball.
The feared strike bowler also effected the run-out of Andrew Ellis. The batsman tried to steal a quick single and Lee, in his rush to gather the ball off his own bowling, collided with Ellis and, from the somewhat undignified position of flat on his back, speared a throw at the stumps.
Otago batsman Aaron Redmond entertained with a 21-ball 50 that featured a couple of towering sixes that must have gone close to 100m. His breezy innings of 58 gave the Volts the momentum they needed after a sluggish start in reply to Canterbury's total of 162 for six.
Ultimately, Otago needed eight runs to win from the last over and it took an inside edge from the bat of Ian Butler and Craig's effort from the one ball he faced.