You have caught me without my old books, but I can tell you the orange wire is for the reversing valve on a heat pump. Almost all manufacturers, except for Lennox, are: R=Power (24volts), G=Indoor fan (Blower), Y=Cooling (A/C), and W=Heat. Some t'stat's are set up to use 2 separate transformers (24 volt), that is why you will see Rc, Rh. This is power for cooling and power for heating. If this is what you have, just take the Red wire and strip it back far enough to go under both screw terminals if the unit is a residential one. I've been doing commercial work for over 20 years and have never seen an "A" terminal, so I can't help you there. You may want to double check that. If I'm reading you correctly, I would put the orange wire on the "O" terminal. Don't understand why the "Y" terminal is not used. It turns on the compressor. The "O" goes to the reversing valve to "Tell" the outdoor unit that it is either in the cooling or heating mode.
Good Luck and let me know if I can help any further.
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I had a Trane Thermostat that had G, O, X2, Y, R, W, & B. The Honeywell RTH6300B says to use G, W, Y, R. It says do not use B, says nothing about X2, and I think I'm supposed to use O but not sure where to put it. I have a heat pump.
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