O'Brien was the New England Patriots offensive coordinator when he took the Penn State job in January, and the school was reeling from the Jerry Sandusky child sex-abuse scandal. He would become the first man not named Joe Paterno to coach the Nittany Lions since 1965.
The program was hit in July by NCAA sanctions that included a 4-year postseason ban, and the team started the season with frustrating losses to Ohio and Virginia.
But since then, O'Brien coached it to an 8-2 record.
According to senior center Matt Stankiewitch, O'Brien just told his team to get that first win and everything would roll from there.
He was right.
"He knows what he's doing," said senior defensive tackle Jordan Hill. "He knows what he's talking about. That's a guy that you have to respect, because what he says usually happens."
O'Brien went from being a coordinator for a Super Bowl squad to what was initially seen as a lackluster hire.
But it didn't take long for him to endear himself to the fan base. He made a record-setter of Matt McGloin, the quarterback who was often chided by fans in 2011. He made Penn State's offense exciting. O'Brien coached aggressively, going for it on fourth down 34 times throughout the course of the season, and was never afraid to get in the faces of officials.
But perhaps his greatest accomplishment was keeping together a team through unprecedented sanctions - and making it more than just competitive. Eight is the highest total of wins ever for a first-year head coach at Penn State.
After Penn State beat Wisconsin on Saturday in thrilling fashion and players sang the alma mater, O'Brien grabbed a microphone and had one thing to say to the fans: "We are . . ."
The "Penn State" response was booming.
"Everything he says for the most part is very right and very true," Stankiewitch said. "And we respect that and we really admire that."
O'Brien hasn't confirmed that he will remain at Penn State, but as he hits the recruiting trail on Wednesday, speculation will surely swirl about his coach of the year credentials. Named to the watch list a few weeks ago, O'Brien has some stiff competition in Notre Dame's Brian Kelly and Ohio State's Urban Meyer, both of whom led their programs to undefeated regular-season records.
It could be tough for O'Brien's credentials to outweigh those of the other two, but that won't change what his players think of him. McGloin says O'Brien helped save his career. Running back Zach Zwinak says he has too many memories of O'Brien to just pick one that stands out.
Thus far in his Penn State career, he's batting 1.000 in having his teams honored on the Beaver Stadium facade, as "2012" is now displayed next to the years of championship and undefeated teams.
Not a bad start.
"I mean, he's been a huge part in this season, obviously, but he's been a big part of a turning point in my life," senior fullback Michael Zordich said. "He let me be the person that I am, on and off the field, and I really appreciate him for that."