In January 1971, O'Neil was appointed to the Boston City Council after the resignation of Louise Day Hicks, who had been elected to the United States House of Representatives.
He was subsequently re-elected fourteen times, each term being two years.

While on the Council, O'Neil thrice ran for Suffolk County Sheriff.
He lost the Democratic nomination to Thomas Eisenstadt in 1974, Dennis J. Kearney in 1978, and Robert Rufo in 1986.

In 1992, he was elected Council President after the death of Christopher A. Iannella.

In November 1999, O'Neil finished fifth (behind Francis Roache, Stephen J. Murphy, Peggy Davis-Mullen, Michael F. Flaherty) in an at-large race in which the top four make the council.
In a story published in The Boston Globe after O'Neil's loss, Boston historian Thomas H. O'Connor wrote, "This is the last hurrah not merely for a man but for the politicking he represents."
O'Connor went on to say that O'Neil's career endured "largely through the kinds of loyalties he built up over thirty years, from people for whom he'd done favors, and they'd never forget him, and they'd talk about him to their relatives.
He built a political career on a system of local patronage."
In January 1971, O'Neil was appointed to the Boston City Council after the resignation of Louise Day Hicks, who had been elected to the United States House of Representatives. He was subsequently re-elected fourteen times, each term being two years.
While on the Council, O'Neil thrice ran for Suffolk County Sheriff. He lost the Democratic nomination to Thomas Eisenstadt in 1974, Dennis J. Kearney in 1978, and Robert Rufo in 1986.
In 1992, he was elected Council President after the death of Christopher A. Iannella.
In November 1999, O'Neil finished fifth (behind Francis Roache, Stephen J. Murphy, Peggy Davis-Mullen, Michael F. Flaherty) in an at-large race in which the top four make the council. In a story published in The Boston Globe after O'Neil's loss, Boston historian Thomas H. O'Connor wrote, "This is the last hurrah not merely for a man but for the politicking he represents." O'Connor went on to say that O'Neil's career endured "largely through the kinds of loyalties he built up over thirty years, from people for whom he'd done favors, and they'd never forget him, and they'd talk about him to their relatives. He built a political career on a system of local patronage."
Dapper O'Neil