O'Neill claimed all honours around him in Ireland at underage levels, as well as playing for Kilkenny U-21 hurley team. He finished second at light-heavyweight at the National Championships 2005 and 2006 behind Kenneth Egan. O'Neill has since won four national middleweight championships and two heavyweight championships where he has excelled. He lost to Darren Sutherland on a close decision in 2008. He also lost out on a chance to compete at an Olympic qualifier to Sutherland in an IABA box-off. O'Neill was also 2009 European Union champion and 2010 European silver medalist. In the 2010 European championships he defeated four opponents including medal favourite Sergey Derevyanchenko of Ukraine before losing to Artem Chebotarev of Russia in the final. He was named the captain of the Irish boxing team at the 2011 European Championships. He secured automatic Olympic qualification for the 2012 London games in the World Amateur Championships 2011. At the 2012 Summer Olympics in London he again captained the Irish boxing team, but went out in the last 16 to Stefan Härtel. He bore the flag for Ireland at the 2012 Summer Olympics closing ceremony. By 2015, he had moved up to heavyweight, reaching the Irish final.