The Hawks gave up their first pick, No.19 overall, in the NAB AFL Draft for the 20-year-old midfielder, who played nine games in two seasons at the Giants during an injury-interrupted start to his career.
"I'm stoked to get the trade done and end up at a great club like the Hawks," O'Rourke told the club's website.
"(It's) a successful club and obviously coming back home I wanted to come to a good club with a good culture and I think Hawthorn's a perfect fit.
"Watching them they're just a classy side and the impressions are pretty high. Back-to-back premiers … it says enough.
"Seeing all the memorabilia and a bit of history to the club is also exciting coming from a new club that hasn’t really established itself yet.
"Coming here it's going to be a big change."
O'Rourke suffered a broken jaw and battled hamstring issues during his debut season in 2013 and played only one match.
He managed eight senior games this year, highlighted by a 21-disposal effort in round 20 against North Melbourne, as well as 10 NEAFL appearances.
O'Rourke's stint at the Giants ended on the sidelines after he pinged his hamstring again in round 22 last season, but he hopes to hit the ground running at the start of pre-season.
The 188cm on-baller said he was under no illusions about the challenge confronting him just to break into the Hawks' powerful and experienced line-up.
He will be fighting the likes of Billy Hartung, Jed Anderson, Brendan Whitecross, Jonathan Simpkin, Angus Litherland, Alex Woodward and James Sicily – none of whom played in the Grand Final - just to break into the Hawks' best 22 next season.
"Definitely, it's daunting," he said.
"They've got that success and that winning culture, something I'm looking forward to hopefully being part of.
"I hope to fit in wherever I can. I can play on a wing or playing inside as well, or even half forward or half-back flank, but I'm pretty happy to play anywhere.
"I'm just looking forward to trying to break into the side."
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