The Red Sox have had a large front office for years now, as have the front offices of teams run by former Red Sox employees. That's why it's not a shock to see that the Sox named both Mike Hazen and Brian O'Halloran (per Alex Speier) to be their new assistant GM, now that former assistant Ben Cherington is the team's general manager.
Hazen was the Vice President of Player Development and Amateur Scouting for the Red Sox, and joined Boston when former GM Theo Epstein got the reins to the club. Before that, like seemingly everyone else promoted in a front office nowadays, Hazen worked for the Cleveland Indians in their scouting department, as the Assistant Director of Professional Scouting, and then Assistant Director of Player Development.
You've heard Hazen's name around here before, most recently in regards to the progress of Ryan Lavarnway's defense, of which Hazen is a believer. He was considered an obvious choice for his new role so long as he didn't head to Chicago with Epstein, from about the moment that it was assumed Cherington would be the one taking over.
O'Halloran was the Vice President of Baseball Operations, but started out back in 2002 as a Baseball Operations Assistant. He followed Epstein and co. from San Diego (as did Hazen, in a way, as he was drafted by the Padres back in 1998), and, like Hazen and Cherington, is well-versed in scouting. He was considered a potential assistant GM for Epstein when rumors of Red Sox personnel leaving town with him, but now he will fill that role for the club he has worked with for a decade.
I leave it to you in the comments to nickname our new two-headed assistant GM for easy referencing.
Peter Abraham has the full breakdown of the Red Sox internal promotions, and with a GM on the way out, there has been a lot of shuffling to make note of.