I am wondering if the airport express station is within working distance to the Mandarin Oriental? If so, is it safe to walk in that area during this time of the night?
You can walk anywhere in Hong Kong safely at any time (for all reasonable definitions of "safe"). And yes it's only 5-10 minutes walk. You can see the obvious route on this map of HK Station and its surrounds:
http://www.mtr.com.hk/jplanner/images/maps/hok.pdf