Character Analysis
Jeremiah T. O'Sullivan, once the lead prosecutor of the Organized Crime Squad, might not be involved in the nitty-gritty of the FBI's deal with Bulger, but the scam would never have survived as long as it does without his support.
After all, he's the first outsider who plops his stamp of approval on the deal. Early on in Whitey's time as an informant, he's accused—rightfully—of being involved in a race-fixing scam, which he denies—falsely—to Connolly. Connolly beseeches O'Sullivan for support, and O'Sullivan's subsequent approval "provided a new layer of protective veneer to the FBI's deal" (1.5.82).
O'Sullivan pops up a few more time in Bulger's tale as a background player, but his key role is as the man who unwittingly helps the whole operation get off the ground. Talk about a dubious honor.
Still, this distance gives O'Sullivan a bit of plausible deniability that neither John Connolly nor John Morris can claim, and ultimately helps him escape stiff punishment.