I came over from Australia 8 months ago. Got to the airport in Oz, was so excited about coming, and lady at check in counter wouldn't let me check in without ticket.
Had to go buy a return ticket on the spot (I just said make it a return for 3 months time), and then go back to check in.
When I got to HK, I had to send the ticket back to a special address for Qantas in Australia, so that the money I spent on the return ticket was refunded.
I NEARLY MISSSED MY FLIGHT BECAUSE OF ALL THAT, SO IT MIGHT BE WORTH CHECKING IT OUT WITH YOUR IMMIGRATION DEPARTMENT AND ALSO HK IMMIGRATION BEFORE YOU COME.