Fellow defender Kendall Waston wasn't even a member of the Major League Soccer club until August.
What a difference a couple of months has made for both players.
O'Brien and Waston form an unlikely partnership in the centre of defence for the Whitecaps, who are riding a four-game shutout streak heading into Wednesday's winner-take-all first-round playoff matchup against FC Dallas.
Like most great pairs, they differ in many ways.
The 35-year-old O'Brien is a no-nonsense veteran of 300 games in the English Premier League, while the 26-year-old Waston was a relative unknown in these parts until arriving in Vancouver from a club in his native Costa Rica.
"They're two opposites," said fellow defender Jordan Harvey. "Kendall's that aggressive one and Andy's that calming influence. Andy's the communication guy in the back and really keeps that backline solid, and Kendall really raises the level of intensity and gets guys up for it."
O'Brien said things have worked with Waston largely because of how well they compliment each other on the pitch.
"I think he's playing very well. I suppose I'm just Steady Eddy and that's how I've played my career," said the former Irish international. "Kendall's aggressive, very dominant, on the front foot. I think when you play with somebody and you know what they're going to do, it's easy to adjust around that player.
"In terms of what you'd expect from a big centre back, he does what it says on the tin."
It often takes time for new teammates to jell and Whitecaps head coach Carl Robinson said before the team left for Dallas on Monday he's surprised how quickly O'Brien and Waston have meshed together.
"Kendall coming in has obviously been a massive hit for the club and for himself, but he's settled in off the field as well. They boys have welcomed him in," said Robinson. "Him and Andy have been fantastic over the last couple of weeks."
Waston scored the goal that clinched Vancouver's second playoff berth in three years on Saturday, flinging himself at a corner kick and powering the ball over the line in a dramatic 1-0 victory over the Colorado Rapids in the regular-season finale.
Waston always has a smile on his face — at least off the field — and said he used to watch MLS games on television back home.
Now he's in the middle of the action for a squad that went an impressive 4-0-1 over its final five games just to qualify for the post-season.
"It's very good that I could come here because this is one of my dreams coming true," said Waston. "I try to enjoy it every day."
He added that he's learned a lot from O'Brien in their short time together with the Whitecaps.
"Andy O'Brien is a guy I want to copy a lot of things from because he's very professional," said Waston. "The success he's had in life is because of that.
"I have a good relationship with him."
Whitecaps goalkeeper David Ousted has had an outstanding season for Vancouver with an MLS-leading 13 shutouts. But he's quick to share the credit with the rest of his team, including the two players who line up right in front of him.
"I think they've been fantastic," said Ousted. "You've seen Kendall, the things he's done just scoring goals and being strong defensively, and Andy has been rock solid the whole season."
Robinson said over the weekend he initially thought Waston would be a good addition for the 2015 campaign, but former captain Jay DeMerit's injury and subsequent retirement forced his hand. O'Brien, meanwhile, got hurt and then fell out of favour in mid-season before battling back to regain his spot ahead of Carlyle Mitchell and Johnny Leveron.
"Age is just a number and I think you need to judged by what you do on the pitch," said O'Brien, who started 11 of Vancouver's 13 clean sheets. "Statistically I'm happy with the games that I've played, the results that we've had."
The defence faces a tough test against Dallas, and O'Brien, Waston and Harvey will be without usual right back Steven Beitashour, who limped off with a hamstring injury against Colorado.
Robinson could opt to go with 19-year-old Sam Adekugbe — who was subbed on in the first half against the Rapids — or 20-year-old Ethen Sampson to fill the void.
The Whitecaps have struggled to score for long stretches this season — they went the equivalent of five straight games without a goal at one point — but the defence has been resolute the last month and was the main factor into the Western Conference's top-5.
"We can talk about that we'd like to score more goals, but it's imperative that you keep a strong backline," said O'Brien. "I think collectively as a team we've done that very well."
Notes: The winner of the Dallas-Vancouver one-game playoff will meet the Seattle Sounders in a two-leg semfinal. ... After Vancouver clinched its playoff spot Saturday night, the players were back on the practice field at 11 a.m. Sunday morning.
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