The 18-year-old centre was selected in the first round of June's draft by Ottawa and will likely spend next season in the Western Hockey League with the Seattle Thunderbirds.
O'Brien was used primarily on the fourth line by the Golden Gophers last season and is anxious to take on more responsibility.
"We want him to get ice time," said Senators GM Bryan Murray. "We want him to develop.
"I never wanted to take a kid out of school. I never encouraged that. I think Jim feels that this is the best road for him at this point in his career."
O'Brien will attend the Senators training camp next month.
After being drafted 29th overall, he attended Ottawa's development camp earlier this summer. He was still recovering from surgery to fix an injury to his left shoulder and was unable to take part in any on-ice sessions during the camp.
O'Brien was the youngest player in NCAA Division I college hockey last season. He had 15 points (7-8) in 43 games.