The 24-year-old Victorian, who's swiftly established himself as a new favourite at Trent Bridge since joining Notts, has been instrumental in helping drive them to joint-top of the first division table with his quick-bowling salvos.
And though he had missed out on taking any wickets in the first innings of their latest home match against Sussex as Josh Tongue and Brett Hutton took the spoils, O'Neill was back at the heart of the action on Saturday as his three wickets helped propel Notts in sight of another victory.
His triple strike in 12 overs that ripped the heart out of Sussex's resistance was headed by an instinctive piece of fielding brilliance off his own bowling when, seemingly a touch off-balance, he stuck out his right hand to snaffle a full-blooded drive from opener Tom Haines when he was going along swimmingly on 64.
The caught-and-bowled came after he had trapped Tom Alsop lbw and before he had Oli Carter caught expertly at slip by England opener Ben Duckett, the three wickets helping reduce Sussex to 7-195 in their second innings, still only 64 ahead with three wickets remaining.
It also brought his haul of wickets for the season to 21, more than anyone else in either division of the championship, but Notts will be disappointed to be losing his services after this match as his deal was just to play the first month of the season to cover for South African Dane Paterson.
Another Australian who's been in good form, 36-year-old Sussex opener Dan Hughes, had earlier been dismissed for 25 by the much-touted English teenage spinner Farhan Ahmed, 17-year-old younger brother of another legspinning prodigy, Rehan Ahmed.
In the second-division clash at Bristol being presided over by two Australian county captains, Gloucestershire's Cam Bancroft and Leicestershire's Peter Handscomb, home opener Bancroft looked set to finally get some runs on the board after a lean match for the pair and Gloucester's big-name Aussie signing Cam Green.
Bancroft knuckled down in the final session of day two to finish unbeaten on 36 as he guided his side to 2-88, only 78 ahead in their second innings after Dom Goodman's maiden first-class five-wicket haul had helped the home side bowl out Leicester for 262.
That 36no represented 24 more than the three Aussies had managed between them in their first knocks, and Bancroft will be looking to kick on towards his second century of the season after his 163 on the same ground against Glamorgan in his opener.
Meanwhile, champions Surrey, who've made a disappointing start to their title defence, established a 38-run lead in their clash with Somerset, with three of their  England batters Rory Burns, Jamie Smith and Dom Sibley making half-centuries as they finished on 7-321 in response to Somerset's first innings 283.
Dan Worrall, Australia's would-be England quick who was named as one of Wisden's five Cricketers of the Year earlier in the week, ended with 2-50 off his 21 overs for Surrey.
