Needing 40 runs for the win with 15 overs remaining, the pair formed a gritty tenth-wicket partnership to seal the victory, with the newly contracted O’Neil contributing a crucial knock of 29.
The teenage allrounder then iced the win for SA with a boundary in the 44th over.
No toss occurred at the beginning of the match, with both teams agreeing that Tasmania would bat first at Maroochydore Cricket Club.
A classy innings from Tasmania’s Corinne Hall, who finished with 97, helped Tasmania to a competitive total of 7/240 at the end of their allocated overs.
The Scorpions’ bowling depth was on show early with five individual wicket-takers for the match, including two apiece for Samantha Betts and former New South Wales all-rounder Ashleigh Gardner.
Requiring 241 for the win, the Scorpions run chase threatened to stall before it started, with Bridget Patterson dismissed first ball.
But SA consolidated shortly after, reaching 2/110 until wickets began to fall steadily.
Captain and opening batter Tegan McPharlin piled on 39 runs, while Amanda Wellington (34), Brittany Perry (38), Alex Price (23) and Tahlia McGrath (31) all made solid contributions with the bat.
But whilst there were plenty of starts a big score wasn’t forthcoming, leaving O’Neil and Harris to chase down the match-winning runs.
The Scorpions will play a Twenty20 match against the Roar on Thursday and another 50-over match on Saturday.