Can you tell me what wires you currently have attatched to what terminals and what empty terminals you have left?
This website should greatly assist you, if you still are not sure what you are doing, just reply back and i will do my best to explain it to you!
O/B is generally the terminal that is for the reversing valve.
Can you please tell me what you have hooked up to what in this format:
Wire Color = Terminal
White = W
This is a regular single stage heat pump system correct? You don't have any backup or aux heat right?
Its best to follow your honeywell instruction manual for installation but i personally think that Orange should go to O/B and W should go to aux or whatever the instruction manual says is the emergency electric heat. I would just double check the White wire was not previously connected to the common terminal on your last thermostat(or C usually)
The air handler is your indoor unit that has your evaporator and your main Fan in it
Your ductwork enters your house from the roof?
Here is a way we can find out what that orange wire is for. Try running your unit without the orange wire hooked up and see what it does, or actually, doesn't do.
Run it in Heating and Cooling mode
Make sure fan operates in both
Also turn emergency heat on and make sure it works
B is used for the reversing valve for RHEEM products and a couple of units by other manufacturers, If there is no B terminal it will be the reverse of O in the set up of your thermostat. If you still need help with this stat reply back and let me know where you are at with it.
I see that both other experts are offline, so i will try and help you out again.
It should be safe to put it on W if you have nothing there and W2 if you do have something there already. Its safe to do this as long as it is not the common side of the transformer