O'Connell's fatal mistake was the Thornton trade, obviously. It's the one and only reason why 18 years later he's never gotten another GM job when the league recycles bad GMs like Don Waddell and Ron Hextall over and over again.
But beyond that unpardonable mistake, in retrospect he deserves more credit than he gets. He helped build two division champs with good moves like acquiring Guerin and then getting Glen Murray in the Allison trade. He brought in depth help like Sean O'Donnell, Dan McGillis, Travis Green. He went for it at the deadline in 2004, getting Gonchar and Nylander. And that's with the firm constraints that Jacobs put on him prior to the 2004-05 lockout (with about half the team on contracts set to expire after the 2004 season). It didn't work out with any playoff success, and Jacobs' plan got absolutely devastated by the NHLPA when they offered the salary rollback as part of the lockout settlement.
His time in Boston ran its course, but I think he could've had a longer career as an NHL exec if he didn't make quite possibly the worst personnel move in NHL history when he rushed to trade the leading point scorer in the league for 3 middling roster players and no futures, without opening bidding to any other team. He did alright for himself in player development though, got his name on the cup in 2014 with the Kings.