I don't know O'Brien that well. Where you see him fitting into this team, 3rd line center?
I'm obviously not the poster you quoted, but I'll answer nonethless.
O'Brien has middle-6 center upside in my opinion, and he'd kind of be in that type of role for the Canadiens if we drafted him and he developed well for us.
Thing is though, O'Brien could also fail to adapt his game to the big leagues and end-up being a AHL/NHL tweener. He's good, and I like O'Brien's game, but he's also far from a sure-thing.
I mean, O'Brien has some interesting upside offensively while being a more-than-decent skater. It also helps that he's overall solid two-way as well and could end up being a strong physical presence in the middle of any NHL lineup though O'Brien isn't inherently that "mean" of a player in the OHL (since he's only 170 or so pounds, and could stand to put up 25+ pounds of muscle on his asparagus-like frame).
The thing is though, if you look at O'Brien's individual offensive skills purely in isolation (playmaking, transition and possession plays, rush offense, sniping, off-puck play, etc.), a lot of his toolset projects as "good, but not great" in the NHL, with no single attribute aside from maybe passing ability that you could definitely say would help O'Brien separate himself from other players.
And then there's also the fact that while I think O'Brien could definitely be a 3C, that his defensive reads and reactions right now don't also project to be anything too special in the NHL barring significant changes.
Simply put, O'Brien's good offensively and generally well-rounded, but for him to make it as a middle-6 center you'd have to bet on the aggregate of these abilities being good enough all added-up for O'Brien to be a good, productive player in the NHL, which could just not materialize.
That's the risk with O'Brien. But one that I'd be more than willing for us to take as the upside could be a good middle-6 player with size and not too many flaws.
But it ultimately depends on who's available when we draft. If a guy like Lynden Lakovic, Justin Carbonneau, or Malcolm Spence is available where we draft, I'd say that we'd need to draft those guys before O'Brien.