Flaghi,
Your HVAC system is a Heat Pump with Auxiliary Heating. This system is compatible with the Nest Thermostat. And, there is a little good news. You have an unused blue wire that can be used for a Power Common. Your HVAC system wires in use are:
Red wire on R terminal, this is Power IN
Yellow Wire on Y, This is Compressor control
Orange wire on O/B, This controls the heat pump Reversing Valve and switch the compressor between Heating and Cooling modes and Vice versa.
Green wire on G, this is the Blower Fan. (Auto/ON).
White wire on AUX, This is auxiliary heat, most likely electric emergency heat strips in the air handler.
Blue Wire, Not currently connected.
Do you have access to a multimeter some times called a Volt Ohm Meter?
Ken, The AC Cooling Wizard
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NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
