The father-of-two joined St Benedict's as a housemaster in 1983. He went on to become head of year, head of sixth form, senior teacher, assistant head and deputy head before taking up his final role as headteacher in 2009.
During his headship he has overseen a long process of reorganisation, welcoming the school's first Year 7 intake in 2013 as it continued to accept students from areas of Suffolk which had already moved to two-tier education.
But as it prepares to become a full 11-18 secondary school from September, when it will start using the closing St Louis Middle School for extra space, Mr O'Neill feels now is the right time to hand over to someone new.
He said: "I always intended to retire at 60, which would have been last year, but because the school's been going through some quite choppy times in terms of reorganisation I thought I should stay in charge until the point where we were moving fully towards being a two-tier school.
"I did wonder whether I should also see through the first year of us using both sites but I decided it made just as much sense for a new teacher to come in and shape that school from the beginning rather than pick it up."
St Benedict's also experienced a difficult Ofsted rating in 2014 when it was downgraded to "requires improvement' and, though Mr O'Neill had hoped it would be re-inspected and upgraded during his headship, he is confident that will happen soon after he leaves.
"I have joked that we're the best performing "requires improvement' school in the country," he said, proud of his pupils' recent academic successes.
One of his proudest legacies is the number of students he has helped get accepted into Cambridge and Oxford Universities. As a Cambridge graduate himself and someone who believes that going to a state school should not disadvantage a person, Mr O'Neill made a commitment to support students' Oxbridge ambitions.
Since he joined the school, up to 50 pupils have gone on to read at Oxbridge ' an impressive record which was recognised by Schools Minister David Laws in 2013. "I'm going to come back and work with the Oxbridge candidates next year, but I'm looking forward to being out of mainstream education for the first time in nearly 40 years," said Mr O'Neill. He is also looking forward to the school's golden jubilee celebrations in January when he has been invited to read during a special mass at St Edmundsbury Cathedral.
In his retirement, Mr O'Neill plans to rekindle his passion for cycling and landscape photography, and to travel with his wife, Elaine, who retired four years' ago. He said it had been a "real privilege' to work with children and to watch them go on to achieve things in life, and that he was grateful to have had such "friendly, supportive' colleagues who had made "going to work any day a pleasure'.
Mr O'Neill will be succeeded by Kate Pereira.
Article and picture courtesy of Bury Free Press