When Bill O’Brien was hired as the 37th head football coach at Boston College, it was welcomed as the start of a prosperous new era at The Heights.

The fanfare has since subsided and the actual job of coaching has commenced. O’Brien initiated the process of establishing a team culture and installing new systems on both sides of scrimmage with the start of spring football on Monday morning inside the Fish Fieldhouse.

“It is really one day at a time and we need to be a hard-working team and be a smart team and we need to be a team that plays with physical and mental toughness,” said O’Brien. “We have to be team that doesn’t beat itself.

“Do a good job at the line of scrimmage staying onside and getting aligned correctly, understanding the call, and executing it correctly on both sides of the ball and special teams. The big thing for Day 1 is to create that culture and it’s started in the weightroom.”

O’Brien replaced Jeff Hafley, who left after four seasons to become the defensive coordinator of the Green Bay Packers. O’Brien and Hafley approach the task of program development from different perspectives.

O’Brien is an offensive mastermind who specializes in developing quarterbacks while Hafley was defense-oriented with an emphasis on the secondary. Both coaches have enjoyed success at the NFL and FBS levels.

While Hafley’s departure was abrupt, he did not leave behind a bare cupboard. O’Brien inherited a largely intact team that won seven games including an 23-14 upset victory over No. 24 SMU in the Wasabi Fenway Bowl on Dec. 28 at Fenway Park.

There was a substantial media presence watching the drills from the sideline on Day 1 and all eyes were on second-year quarterback Thomas Castellanos, a transfer from Central Florida who emerged as one of the most dynamic dual-threat field generals in the Power-5.

In 12 regular season games and 11 starts, Castellanos led BC in rushing with 1,113 net yards on 215 carries and 13 touchdowns. He completed 189-of-330 passes for 2,248 yards and 15 touchdowns with 14 picks. Castellanos’ 3,361 yards of total offense are third most by a BC quarterback behind Matt Ryan and Doug Flutie.

Castellanos is described as a runner who can throw, but O’Brien feels he can become the complete package.

“He’s really embraced the system and he’s worked very hard and he’s taking it one day time and he’s trying to get better every single day,” said O’Brien. “But he is definitely a guy who can do both, no doubt about it and I think he is a really good football player and I’m glad we have him.”

There was speculation that Castellanos would enter the NCAA transfer portal is search of a better playing opportunity with greater NIL potential to cash in on his skill and name recognition. The transfer rumor sprouted legs after the Eagles’ win over SMU, where Castellanos rushed for 155 yards and two touchdowns under blustery raw and wet conditions. But Castellanos insisted after the game that he would return to BC and that was long before O’Brien entered into the equation.

“I always knew I was coming back,” said Castellanos. “These guys gave me the keys to this place with coach Hafley and Boston accepted me as family in their community. These people have treated me very well and I decided to stay.”

Castellanos did thrive in 2023 in spite of patchwork scheme devised by co-offensive coordinators, Steve Chudzinski, and Steve Shimko.

O’Brien and offensive coordinator Will Lawing have installed a multiple pro-set offense that will look to tap into Castellanos’ creative aspects. O’Brien wants to incorporate some of what he did for Bill Belichick with New England and Nick Saban at Alabama.

“We will do a lot of different things and be multiple in our personnel groups and multiple in our schemes and we will take advantage of all the things he can do.

“I think that is the key. That is the type of system we run, a quarterback-centric system on who the quarterback is what that guy can do and he can do a lot.

“With all the guys I’ve worked with, the most successful guys had anticipation and the ability to get it out quick. Good decision makers that understand the rush is coming.”

O’Brien worked with Tom Brady in New England and Deshaun Watson as the head coach of the Houston Texans. At the college level, he helped develop Bryce Young into a Heisman Trophy winner and made Mac Jones the Patriots’ 2021 first-round draft pick. Castellanos bought in on what O’Brien was selling after their first meeting.

“I am seeing a lot of passing the ball, a lot of trying to get explosive plays throwing the ball down field,” said Castellanos. “We have a lot of good receivers and I am trying to get the ball in their hands to make plays.

“I feel this year will be more passing and my developing and being more polished. When we got him, I was excited because he worked with great quarterbacks in the NFL and college.”