The promising striker pens a three-and-a-half-year contract with the Cherries despite reported interest from clubs in the Premier League.
Bournemouth boss Eddie Howe told the Daily Echo: "He is not necessarily going to be involved straight away, which he is aware of. But with a lot of hard work and a good attitude, we could have a really good player on our hands in the not too distant future.
"We are excited by him and are looking forward to working with him but don't want to put too much pressure on him too soon. It is a big jump so we are going to need a bit of patience from everybody."
O'Hanlon, 18, had a trial with Manchester United ahead of his move to the Goldsands Stadium.