I'm trying to replace an old Dayton thermostat (Model 2E137) with a newer Honeywell (either the 6300B or the 7400). I have a Bryant 80 furnace in the basement and a Bryant Quantum Plus 550A air con outside. The Dayton thermostat has wires going to six terminals:

R, 4, Y, W, G, and O

I know that Y, W, and G go to the same terminals on the Honeywell, and I suspect that R should go to Rc and 4 should go to Rh. But what do I do with the wire currently going to O? I don't have a heat pump -- or least I don't think I do given the components I noted above.

Was O hooked up on the old Dayton thermostat? If so where does it go on the equipment?