There are 2 places where you can buy the O-card at the airport. They are on the lower level on opposite end of the terminal.

$300 HK per card. That will get you round trip to and from HK Central plus 72 hours of unlimited use of the MTR. At the HK Central station free shuttles are provided to most major hotels, including the Conrad - I believe it is Shutte #2.

When you finish your visit to HK you can take the card back to where you bought it and get a $50HK refund. This makes the card at $250HK; a bargain in my opinion. The $50 got me a last minute meal at one of many fast food places.

The Airport Express is fast (30 minutes) and dependable; fun for the kids too.