This is an article from last year that attempts to trace coaching philosophies/lineage in the NBA. Probably too long to post in its' entirety here, so I will just highlight the O'Brien bits. It's worth reading the whole thing though, I learned some interesting things about the relationship history of some of these coaches and think some of the analysis here is spot on, though in some places the author seems make more of a leap than he should, such as when he puts O'Brien in the Ramsay tree simply on the strength of their family relationship.
Rick Pitino Coaching Tree
Trademarks: Heavy usage of three-pointer, pressure defense
Current Head Coaches: Jim O'Brien, Reggie Theus
Other Notables: Billy Donovan, Tubby Smith
Pitino's coaching tree is much more impressive at the college level, where Donovan and Smith are two of many former assistants who have gone on to run their own teams. In the NBA, Pitino's style has been effectively copied by O'Brien, his assistant and successor in Boston. O'Brien's teams, like Pitino's, are known for taking a ton of threes, and his defenses generally employ heavy trapping if not as much full-court pressure. In his second NBA season, Theus--a Pitino assistant at Louisville--has yet to firmly establish his coaching philosophy, but the buzzword seems to be "aggressive," which is very much in keeping with Pitino's influence.
Jack Ramsay Coaching Tree
Trademarks: Measured approach
Current Head Coaches: Rick Adelman, Jim O'Brien
Adelman Sub-Tree: Terry Porter
Dr. Jack continues to wield influence in the league a full two decades after he last paced an NBA sideline. O'Brien is the first coach to make an appearance in two coaching trees. Stylistically, he belongs much more in the Pitino tree, but it's hard to deny his relationship with Ramsay--his father-in-law. Adelman started his NBA coaching career as an assistant to Ramsay in Portland, and can trace his even-keeled style to his mentor. In turn, Adelman influenced long-time Blazers point guard Porter, who spent his first year on the sidelines assisting Adelman in Sacramento.
Jim O'Brien - Rick Pitino, Jack Ramsay. O'Brien is one of the most orthodox disciples you'll find, his style (threes and traps) heavily influenced by Pitino after several years together first at Kentucky and later with the Boston Celtics. Ramsay has surely had an influence as well as O'Brien's father-in-law.