All thermostats may or may not have these designations. They are the following...
R hot 24V
G blower
Y1 1st stage cooling
Y2 2nd stage cooling
W1 1st stage heat
W2 2nd stage heat
O reversing valve
C common
E emergency heat
You didn't mention if you have a heat pump or natural gas heat. If a heat pump be aware that the O designation can be programmed/wired to either energize or de energize the reversing valve in heat mode. If you have a heat pump it can be tricky if your not familiar with such logic. If you have straight cooling with a natural gas furnace it's a lot easier.
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