His instincts told him to charge. But he was constantly being told to hold back, don't take any chances, don't make any mistakes.
That's why O'Brien is thankful for the change in style since being traded from the Ducks to the Tampa Bay Lightning last week.
"The way they play is two different ends of the spectrum," the rookie from Port Hope, Ont., said Monday after the Lightning practised in preparation for Tuesday night's NHL game against the Vancouver Canucks.
"In Anaheim, for me and my partner, it was chip it out, put it off the glass. They didn't really want us jumping in at all."
The Lightning have eased the reins on O'Brien. Coach John Tortorella wants him to join the rush, take a chance and make an offensive play.
"We knew he had the ability to jump in," said Tortorella. "We're in the process of trying to teach him how we play.
"It's a little bit different than where he played before. We expect our defence to jump in. If they're not jumping in, we feel they are not playing under our team concept."
O'Brien said with the Ducks he was not supposed to go below the top of the offensive zone circles. Coach Randy Carlyle didn't even like him to skate with the puck.
The change in philosophy in Tampa Bay is like being able to listen to rock guitars again after being fed a steady diet of classical violins.
"No disrespect to the way Randy Carlyle coaches, that's just not me," said the 23-year-old. "I never really felt like I was doing anything, just chipping it out.
"If I didn't make a mistake it was a good game. It didn't matter if I made zero good plays. Torts (Tortorella) has been great. He said 'Don't overthink it. We just want you to play.' "
Tortorella expects O'Brien will get caught out of position some nights.
"This is a game of mistakes," he said. "If you continue to make the same mistakes constantly that's when I think we get a little concerned.
"You have to let him progress."
The six-foot-two, 237-pound O'Brien also brings some toughness to the Lightning. He massed 319 penalty minutes in 77 games with Cincinnati of the American Hockey League in 2004-'05.
That's an insurance policy Tampa's Tim Taylor likes to have.
"I like the fact he is very willing to stick up for teammates right away," said Taylor. "We needed some team toughness.
"Shane O'Brien is going to be a key factor in how far we go (in the playoffs)."
Anaheim selected O'Brien 250th overall in the 2003 draft. He spent three years in the AHL before joining the Ducks this season.
O'Brien was traded to Tampa along with a 2007 third-round draft pick for goaltending prospect Gerald Coleman and the Lightning's 2007 first-round pick.
In 65 games this year, he's got two goals, 13 assists and 144 penalty minutes. He has averaged just over 14 minutes of ice time.
In the three games since coming to Tampa, O'Brien has picked up one assist and seen his ice time increase.
With the Ducks having both Chris Pronger and Scott Niedermayer on defence, O'Brien understands why his ice team was limited.
"It didn't matter what I did," he said. "I could have played the greatest game of my life and still play the same amount of minutes.
"They have two of the best D-men in the league. I learned a lot playing there. They are both great guys. This is a better fit for me."
After a slow start, Tampa Bay is battling Atlanta for first in the Southeast Division of the NHL's Eastern Conference. Both teams have 78 points.
The Canucks have 81 points, giving them a two-point lead over Calgary and Minnesota for first in the Northwest Division of the Western Conference.