Long before Harry O'Brien pulled on a Collingwood jumper, he knew AFL football would play a big part in his life.

The Collingwood defender, who this week chalks up his 150th career game at the age of 25 and in his 12th season, had a clear vision of what he wanted to do with his life as early as 14 years old.

He'd been involved in football since dabbling in Vic Kick [now Auskick] when he was six and living in Eltham, Victoria, and then playing under nines for Rossmoyne Football Club in Perth.

As an Essendon supporter, he'd been a big fan of Michael Long for what he could do both on the field and off it.

And so, he came to the decision a career at the highest level was what he would diligently work towards.

"My dream started to become a reality when I got to about 14 or 15 and I knew I'd become an AFL footballer," O'Brien said this week.

"I knew it was my destiny.

"I saw the impact someone can have when they're in the spotlight. I knew my biggest passion beyond football was being able to help other people, and that's why I was so driven.

"I knew it was my destiny if I was able to use the spotlight of being a professional footballer, I could combine that with my biggest passion."

It didn't come easily for O'Brien. He was physically gifted, and had a level of fitness honed from his days doing Little Athletics as a youngster.

But his decision-making ability and disposal contributed to him being overlooked at the 2004 NAB AFL Draft.

He turned his attention to the rookie draft the following month, and decided he would do everything he could to make sure a club called his name.

It's been well documented how O'Brien paid his own way to Melbourne after contacting Collingwood and asking if he could train with the club ahead of the rookie draft.

"I remember thinking, 'Well, if this is the only two weeks that I get to feel as though I'm an AFL player, I really want to make the most of it and really get out of my comfort zone', and that's what I did," he said.

"I'm a great believer in that it doesn’t matter how you get your foot in the door in a place - once an opportunity arises, it's an even playing field.

"It was the first time I'd set my mind to something, followed it, a process, and I was able to finally get the result I wanted to get.

"That's really assisted me with my life; not just on the field, things off the field as well."

Since coming to the club in 2004, O'Brien has faced challenges on and off the field, none more so than when his stepfather Ralph passed away suddenly in Perth in 2009.

He embraced meditation as a way to "be proactive about his mental state" in the time following, and described the 2010 premiership as "absolute bliss" after such a dark time.

 "For me, on a personal note, the pain that I felt in 2009, how hard that year was for me, 2010 was finally a ray of light that I could give to my family, and for myself," he said.

"From a year where you were struggling and questioning your motives to live, really … it was a year that just really, to be self indulgent, was bliss.

"It's why everyone chases it. Everyone chases that bliss in some form, some way or another.

"I'm fortunate I got to achieve it in a football sense."

O'Brien hasn't decided what he wants to do when he stops playing football, but knows it will be something that makes him happy.

But if the club's sports science director David Buttifant is on the money when assessing O'Brien's physical state, he doesn't have to worry about making a decision just yet.

"I asked him, how long do you reckon I've got? He reckons I'm the type of player and with my body I could play until I'm 33," O'Brien said.

"Honestly, there's a lot of things that could happen between now and then.

"I'll play for as long as I can."

So does playing 150 games mean more to O'Brien because of the way he had to work for his career to even get off the ground?

He doesn't think so. But he is proud of the fact he's had the determination and commitment to make it to this phase of his AFL dream.

"It was something I worked hard at, and it was a dream I consciously created for myself," he said.

"It's always pleasing when you set out to do something and you're able to achieve it.

"When I think about it, it's quite a lot of football.

"I'm really proud to have been able to make the most of the opportunities that I've been able to create and the football club have given to me."

Jennifer Witham is a reporter for AFL Media. Follow her on Twitter @AFL_JenWitham

The views in this article are those of the author and not necessarily those of the club