Your thermostat uses only 4 wires and you have a Heat Pump. If you notice the Yellow Wire is connected to W then Y. So, when you choose Heat on the thermostat, the yellow wire is activated and that turns in the compressor. The white wire is attached to O terminal I think. In the air handler wire terminal strip, is is definitely attached to the Orange control wire going to the reversing valve coil. Notice you have no auxiliary heat (W1 or W2).
If it were me, I would follow industry standards and use the blue wire and attach it to the C on the terminal strip, take the orange wire and attach it to O and disconnect the white wire on the terminal strip.
With your Nest Thermostat you will attach Red wire to R/Rc, attached the Orange wire to O/B, attach the Yellow wire to Y/Y1, attached Green wire to G, the Blue wire to C.
You can use either the Nest Thermostat or upgrade to the feature rich Nest Learning Thermostat.
AC Cooling Wizard
