He started off with a narrow 20 point win over Steve Smith before scoring 96 against Peter Wallis. All of his next 5 games were centuries including a high score of 114 against Theresa Turrell. In his final game, he dropped to 94, but still qualified for the finals as #3 seed with 804 points.
In the quarter-finals, he played Judith Young in a close game. The game hinged on O'Neill making 209 from 75, 100, 25, 6, 10, 50, which involved him doing 50 * 100 + 75 to make 5075 before coming back down again (although it can also be solved by doing 100 + 75 + 50 - 10 - 6). Though Young beat him with SEARCHED in round 12, O'Neill won by a score of 90 – 74, where failing to unravel the conundrum IRONCASTS separated him from a score of 100.
In the semi-final, he faced John Mayhew, the No.2 seed with 811 points. O'Neill led by 7 points going into round 13, where he was beaten by THEOREMS and trailed by 1 point going into his choice of numbers. Controversially, he didn't choose 4 large and went for just 1 large number. Both players scored 10 points and the lights were dimmed for a crucial conundrum (EROGENICC), but neither player could solve it and Mayhew won the game by a solitary point. It was the second conundrum of French origin in a row that O'Neill had failed to solve.
One year later, O'Neill was invited back to the Championship of Champions XII, and faced Series 52 octochamp Jack Welsby. O'Neill started badly missing AMOEBAS. O'Neill couldn't beat Welsby in any letters round, and uncharacteristically was beaten by Carol in both choices of 4 large numbers. O'Neill did get the conundrum PROFANITY but it was Welsby that won the game by just 5 points.
O'Neill returned to Countdown in 2013, as one of 41 contestants taking part in the 30th Birthday Championship. He received a bye to the first round, and won his first game against Series 59 semi-finalist Kai Laddiman by a score of 101 – 76. O'Neill then beat Series 6 champion Darryl Francis, scoring a personal best of 122 in the process. He then equalled his personal best in his quarter-final against the Series 66 champion Jack Worsley, and in the process, he achieved a max game, becoming only the third person in the history of the 15 round format to achieve this feat. In the semi-final, he played the formidable Series 54 champion Conor Travers. He managed to hold Travers for nearly every round, but his gamble of SULPHATED ☓ in round 6 backfired, and Travers solved the conndrum INFIGHTER in one second to achieve an unprecedented third consecutive max game. O'Neill’s semi-final spots make him among the strongest players to never win a series.
Since appearing on Countdown, O'Neill has played at a number of unofficial Countdown tournaments, including COLIN 2005, CoBris 2005, COLIN 2006, CoBris 2006, COLIN 2007, CoNot 2008, CoNot 2009, COLIN 2010, Co:Lon 2010, COLIN 2011, CoNot 2011, Co:Lon 2011, CobliviLon and COEDI. He achieved 3rd place at CoBris 2007, 2nd place at COLIN 2008 and COLIN 2009, before winning his first event CoLei2 later in 2009. In 2008, he also appeared in the Countdown Online Masters via MSN Messenger, and lost 100 – 99 to Series 47 champion Chris Wills in the quarter-final.
In 2009, he became the first moderator of the Off-topic sub-forum on c4countdown.
His attendance (and of course, victory) at CoLei2 2009 has caused one c4c forum member to confess to another to have considered him to be the 'most attractive man in the room', a quality not diminished by Jon later putting an empty 'Celebrations' tin on his head. He has a face capable of carrying off such a jaunty appendage.
On 19 September 2014, he was a special guest on 8 Out of 10 Cats Does Countdown Series 5, Episode 3. Jon joined Sean Lock and Jo Brand's team. Unfortunately, he missed a numbers target to Jon Richardson, saying it was hard to concentrate, but did redeem himself by solving the conundrum SNOGAMILF in under a second.
|#||Date||Type||Contestant 1||Score||Contestant 2||Guest||Lex||Max|
|3898||1/04/2005||P||Steve Smith||67 – 87||Jon O'Neill||Mitch Murray||Susie Dent||143|
|3899||4/04/2005||P||Jon O'Neill||96 – 72||Peter Wallis||Nicholas Owen||Susie Dent||120|
|3900||5/04/2005||P||Jon O'Neill||106 – 54||William Moore||Nicholas Owen||Susie Dent||140|
|3901||6/04/2005||P||Jon O'Neill||102 – 46||Ola Odutola||Nicholas Owen||Susie Dent||124|
|3902||7/04/2005||P||Jon O'Neill||101 – 61||Gill Johns||Nicholas Owen||Susie Dent||133|
|3903||8/04/2005||P||Jon O'Neill||104 – 66||Phil Squires||Nicholas Owen||Susie Dent||119|
|3904||11/04/2005||P||Jon O'Neill||114 – 43||Theresa Turrell||Julia McKenzie||Susie Dent||132|
|3905||12/04/2005||P||Jon O'Neill||94 – 58||Gary McEnaney||Julia McKenzie||Susie Dent||118|
|3955||28/06/2005||QF||Judith Young||74 – 90||Jon O'Neill||Philip Franks||Susie Dent||121|
|3958||30/06/2005||SF||John Mayhew||89 – 88||Jon O'Neill||Philip Franks||Susie Dent||117|
|4116||1/06/2006||CP||Jack Welsby||99 – 94||Jon O'Neill||Ron Atkinson||Susie Dent||134|
|5625||21/01/2013||30B1||Kai Laddiman||76 – 101||Jon O'Neill||Margaret Mountford||Susie Dent||132|
|5641||12/02/2013||30B2||Jon O'Neill||122 – 91||Darryl Francis||Ken Bruce||Susie Dent||130|
|5649||22/02/2013||30BQF||Jon O'Neill||122 – 104||Jack Worsley||Dr Phil Hammond||Susie Dent||122|
|5653||28/02/2013||30BSF||Jon O'Neill||102 – 120||Conor Travers||Jenny Eclair||Susie Dent||120|
& Sean Lock
|52 – 24||Jon Richardson,|
& Rob Beckett
|Adam Buxton||Susie Dent||69|