The 24-yea-old Ireland International is strong in both attack and defence and becomes Hull’s fifth signing ahead of the 2012 campaign.
Speaking on the deal, O’Carroll said: “To be joining a club like Hull FC is massive. They are a huge club with a great history and a great fan base and I am very privileged to be joining them,” he said.
“I am looking forward to getting over there and getting into pre-season and it is a move that really excites me, so I can’t wait to get started.
“They are a club with big ambitions and hopefully I can play my part in helping them achieve some of those.”