COLLINGWOOD defender Harry O'Brien labels it a "great achievement", something that has "come along pretty fast", and a milestone he's "simply grateful" to be given the opportunity to reach.

Ultimately, for the Brazilian born 21-year-old, he hopes that playing his 50th game this weekend is the start of something even greater than racking up a half century of senior appearances.

"It still feels like yesterday that I was on the rookie list and hadn't played a game yet, so it has gone pretty fast," O'Brien told collingwoodfc.com.au, ahead of the Pies' clash with the Sydney Swans on Saturday.

"It's only a very small milestone but I'm very happy to have played 50 games of AFL football and hope I can keep going.

"It feels like I've played more than that because I was a rookie and I got elevated and then had to go back.

"But to me, it's more about it being the beginning of something that I'll hopefully be around to achieve over a long time.

"The fact I've played 50 games gives me a lot of faith in myself to know that I can keep going on and play more games of football. I'm really rapt to get there."

For O'Brien, there was always little doubt in his mind he would gain an opportunity to become an elite footballer, owing to a belief he had the ability to play at the highest level. 

"It's been something I believed I could do from a very young age, and for as long as I can remember," he said.

"I always believed that I could play AFL football, no matter what route I took and no matter how I got there.

"I knew I could make it a career, and a long career at that."

To coincide with O'Brien reaching his first games milestone, the club recently announced it had penned a new three-year deal with the defender, which will ensure he stays a Magpie until at least the end of 2011.

The former rookie said the lengthened contract has given him further belief in his abilities, and will help to achieve his goal of staying a Pie for the duration of his playing days.

"I'm just extremely grateful to the club that they've shown faith in me, and it gives me a lot of confidence that I can play my best footy in this environment because they've been so great," he said.

"I'm just so grateful and I thank God that I've been able to get this opportunity.

"They've shown faith in me to offer me three years, and I really want to play the rest of my career at Collingwood so it gives me that opportunity, and an opportunity to be there for the next three years is fantastic."

He also said he is excited about remaining an important cog in the Magpies' young defensive structure.

"I'm really looking forward to what the future holds, with the young group that we have, and the young defenders group that I'm a part of," he said.

"The other boys like Nathan Brown have signed on as well, and we're the future of the backline, and it would be really good to play a lot of football with these guys."