O'Sullivan died in 2009. J.W. Carney Jr., a member of Bulger's legal team, asked U.S. District Judge Richard Stearns to recuse himself Wednesday, saying he was close to O'Sullivan and could be called as a defense witness, the Boston Herald reported.
Stearns denied an earlier recusal motion, saying in July he is confident he can remain impartial. He worked in the U.S. attorney's office with O'Sullivan but was not part of the Organized Crime Strike Force.
Bulger was arrested in June 2011 in Santa Monica, Calif., where he had been living for years under an assumed name. He faces a long list of charges and is accused of many killings.
Starting in 1971, the FBI used him as an informant and agents shielded him from arrest.
O'Sullivan testified under oath before Congress that he was not involved in any grant of immunity to Bulger. Carney has said he is also considering calling FBI Director Robert Mueller as a witness and argues Stearns cannot be impartial about deciding whether he or Mueller should testify.
"Federal law mandates in this situation that Judge Stearns recuse himself from this case," Carney said in his motion Wednesday. "The law -- and common sense -- says that a person cannot be both judge and witness."