The Volts restricted the home side to 124 for five, all-rounder James Neesham starring with three for 11 before scoring a hard-hit 57 in the successful chase.
Left-arm spinner Nick Beard was miserly, conceding just 16 runs from his four overs, and Neil Wagner took one for 15 from three overs.
Derek de Boorder whacked 33 and Neil Broom (19) and Nathan McCullum (18) made useful cameos in the chase.
Otago made first use of the surface with Jacob Duffy enjoying early success, chipping out the first wicket before the opposition got into double figures.
An inspired spell from Neesham and good control from Beard kept the pressure on the Sri Lankan batsmen through the middle stages.
The Sri Lankans finished with a flurry, though. Shalika Karunanayake made 64 and was the pick of the batsmen.
Otago pace bowler James McMillan and spinner Mark Craig played for the opposition and McMillan struck first for the local team when he had Hamish Rutherford caught at third man with the score on eight.
De Boorder and Broom added 31 for the second wicket and helped set a solid platform.
Neesham's fine innings of 57 came from 36 balls and got his side into a commanding position.
The win was achieved in the 17th over but the full 20 overs were bowled to give Aaron Redmond and Michael Bracewell some time in the middle.
The two sides will play again today.
