Dave was a Cub, Scout, Venturer and Rover. His father Kevin is still the Group Leader at the 1st/3rd Mitcham Scout Group. After finishing high school, O'Neil completed a course in primary school teaching; however, he never taught. He became a field officer for the Red Cross, giving talks and training sessions, where he first enjoyed public speaking and the opportunity to tell jokes. In the late 1980s, he was a member of Melbourne band Captain Cocoa, in which he played bass. His identical twin brother, Glenn, was the lead vocalist. He resides in Melbourne.
Radio career
O'Neil ventured into radio in the early 1990s, appearing on the Osso Booko Show on Melbourne community station 3RRR from 1992 to 1997. He co-hosted the one-hour sketch comedy show on Sundays with Vic Plume and Alan Parkes. He also spent some time on the RRR Breakfast team with Kate Langbroek and regular phone-ins from Dave Hughes. In 2001 he joined then-new radio station Nova 100 in Melbourne, on the top-rating Hughesy, Kate & Dave breakfast show. He later left in July 2006 for Nova's sister station, Vega 91.5 to co-host the breakfast show Dave and Denise with Shaun Micallef. In 2007, Ian "Dicko" Dickson and Chrissie Swan joined the show with Denise Scott with Shaun Micallef leaving the station. In 2010, once Vega 91.5 became Classic Rock 91.5, Chrissie Swan departed the breakfast team. However, on 15 July 2010 Ian "Dicko" Dickson and O'Neil were axed from Classic Rock 91.5.
In 2010, O'Neil features in a cameo performance in the upcoming Australian mockumentary musical film Ricky! the movie.
Television
O'Neil has appeared on Under Melbourne Tonight, The Loft Live, Spicks and Specks, Sleuth 101, Adam Hills In Gordon Street Tonight, Tractor Monkeys, All Star Family Feud, Celebrity Name Game, The Project, Show Me the Movie!, Have You Been Paying Attention? and Hughesy, We Have a Problem.
In 1997, O'Neil featured on the RMITV show Under Melbourne Tonight's 1997 Christmas Special.
From 1999 to 2000, O'Neil also featured on the RMITV show The Loft Live on 8 July 1999 and again on "The Loft Live Comedy Benefit" episode on 10 February 2000.
From 2005 to 2014, he also appeared frequently on the Australian music TV show Spicks and Specks.
In 2010, O'Neil also appeared on the first episode of ABC 1 miniseries Sleuth 101, in which he was a guest detective, solving a murder mystery.
From 2011 to 2013, he also appeared regularly on the ABC 1 talk showAdam Hills In Gordon Street Tonight.
In 2013, he appeared on the ABC's Tractor Monkeys.