And while the Victorian prospect was delivering again with both bat and ball for the champs against leaders Essex at Chelmsford on Saturday, incumbent Test allrounder Beau Webster was impressing in similar fashion in Warwickshire's title charge.
O'Neill only had a short time to impress at Trent Bridge last year but his early-season stay in Nottingham proved hugely significant as his contributions helped them lift the title for the first time in 15 years.
This season, the 25-year-old Melburnian has played five first-class matches and his efforts have proved even more impressive, as he's averaging 57 with the bat and 17.34 with the ball, the best by any bowler who's delivered 75 overs or more in the championship's first division.
Saturday offered another glimpse of his worth to Notts, who are fifth in the table but only five points off the lead as, first, coming in at No.8 he smashed 61 off 50 balls, including two sixes and nine fours, pushing the visitors' first-innings lead to a demoralising 273 after teammate Jack Haynes had hit 137no.
Then he emerged to reduce the leaders to 2-13, bowling opener third ball before also getting nightwatchman Jamie Porter caught at third slip.
While O'Neill's been tearing it up with Nottinghamshire, Webster has been a little critical of his own contributions at Warwickshire in the championship, even though he batted brilliantly in their recent T20 Blast matches.
But in what's set to be his last first-class appearance this season, the 32-year-old Tasmanian has found his best form at Taunton, following up his 4-23 that helped bowl out Somerset for 208 on Friday by cracking a 101-ball 64, with eight fours and a six, that helped Warwickshire gain a first-innings lead of 122.
Championship story of the day, though, was the blistering return of Ben Stokes to county action while his England charges were toiling against New Zealand at the Oval.
Stokes, left out of the second Test pending the outcome of an investigation into an incident at a London nightclub, blasted 95 off 118 balls for Durham against Northamptonshire, narrowly missing what would have been his first first-class century in 11 months.
It was an Australian who spoiled the script as Harry Conway (3-47) bowled the left-hander after Stokes had smashed 18 boundaries.
