The 20-year-old defender made his first-team breakthrough in October and was offered a new deal after an outstanding campaign that saw him make 38 appearances for Stuart McCall's side.
O'Brien said: "I would love to have another season just like I've had. I look back on last year and it was incredible. But it's like another chapter now.
"I've had four or five weeks rest up and think about everything and now it's all starting again with pre-season on July 1.
"The wingers I was up against before maybe looked at me as a young kid who'd just got into the team. Now I've had a full season it might be completely different.
"I've got to keep working hard and doing well and then hopefully I will keep my place. It would be great if I was given the number three shirt. That would mean a lot to me."