But fully recovered and now restored to the Ireland captaincy under new head coach Joe Schmidt, O'Connell will lead the men in green against the Wallabies on the weekend.
And the 34-year-old admits that the Wallabies will be no pushover under their new boss Ewen McKenzie.
"They are certainly a better team and they have a different philosophy," he said.
"(Quade) Cooper has been brought back in and has been given the opportunity to play the way he wants to play.
"Australia are coming off a really tough Rugby Championship and when you have tough championship like that you tend to find out a lot more about yourself and you tend to get better.
"I think it's going to be really tough in the scrum , I don't think it's going to be anything like the third test against the Lions."
O'Connell came off the bench in last weekend's win over Samoa but will start his first game for Ireland since the 2012 Six Nations on Saturday.
And the Munster legend is itching to make up for lost time.
"It's been a frustrating couple of years. I was a long time away from the Irish setup but it's great to be back but it's been a really enjoyable time, the way we've trained and the way we prepared," he added.
"The squad has changed dramatically since the last time I was involved 18 months ago in terms of the depth of the squad.
"There is a lot if depth in leadership in the squad with provincial captains and former Irish captains in the squad.
"Any time you can have more leadership on the field is so important particularly against these Southern Hemisphere teams."