The 23-year-old O'Brien, who came through the ranks at Bradford and last season made 31 appearances for the Kingstone Press Championship outfit, has signed a three-year contract with their Betfred Super League neighbours.
O'Brien said: "I'm really pleased to have signed for the club.
"I wanted to get back in Super League, I wanted to try with Bradford Bulls but that hasn't happened and with what's gone on at the club in the last few months I just felt like I needed a change for me and my career, for my family and personally it is a good move for me and I'm looking forward to the challenge at Huddersfield.
Been at a club all my life, and today found out it all has come to… https://t.co/2IDOPYHJ1J— Adam O'Brien (@adamob93) January 3, 2017
"I want to say a massive thanks to Bradford, I have been there since I was 15 and brought through the ranks from academy through to first team and I would just like to thank the coaching staff and everyone involved at the Bulls, especially the fans that have been there since day one supporting me."
O'Brien will plug the gap at the John Smith's Stadium created by the departure of Kyle Wood and enable captain Ryan Hinchcliffe to make the switch from hooker to loose forward.
"We now have two young dynamic hookers in Adam and Kruise (Leeming) and that is encouraging for us and the fans and I think they will really enjoy watching their combination," said Giants coach Rick Stone.
"Adam came into training today and I walked away impressed. The squad is starting to take shape now, it has taken a while but we have brought in a lot of good young players and I'm looking forward to seeing what they bring to the team in 2017."