William S. O'Sullivan (January 31, 1928 – March 28, 1971) was an American loanshark and longtime enforcer for Donald Killeen, the head of the Irish mob in South Boston, Massachusetts.
O'Sullivan also acted as a mentor to a young James J. Bulger, who grew up to become the leader of Boston's Winter Hill Gang.
She would later say that William was a "quiet man", a "fine husband" and a "wonderful father, and that he (wasn't) the type of person that you would associate with gangsters.
"[1] Contrary to statements made by his wife, FBI documents report that O'Sullivan had a reputation as a "tough guy" and a prosperous loanshark because he was a close associate of Stephen Flemmi and Frank Salemme.
[citation needed] After Salemme and Flemmi fled Boston as fugitives from murder convictions, rival mobster Donald Killeen wanted O'Sullivan to align himself with Ilario Zannino and himself.
However, Killeen had begun having problems with the Mullen Gang, a group of burglars and stick up men led by Paul McGonagle and Patrick Nee.
In February 1971, Buddy Roache, a close associate of McGonagle told Whitey Bulger and O'Sullivan that he was going to murder Donald Killeen, and if they interfered, he would kill them.
Following the shooting, both Bulger, O'Sullivan, and neighborhood boss Donald Killeen expected retribution for the attack on a ranking member of the rival Mullen Gang.
[3]According to the memoirs of Mullen leader Patrick Nee, Paul McGonagle was thoroughly livid over the murder of his brother and, believing that O'Sullivan was responsible, received permission to kill him.
Heading home, He was chased by a couple of men with the Mullens gang who caught up with him when he tripped over a manhole cover about 200 yards from his house and fell down.
After a sit-down in Boston's South End, the Mullen and Killeen gangs joined forces with Howie Winter as overall boss.