Nest thermostat 3rd gen installation

[Val6]

Hello google community. 
I have recently bought the nest learning thermostat 3rd generation, and wanted to change the old thermostat (pro 621-2) in my house. 
before this I had Maintence on my hvac system (have a heat pump) and the rep confirmed that it’s possible to make that change 

when today I looked at the wires I believe everything was understandable except of O wire (it’s blue) and there is no C wire. Started to read all the articles and realized that i don’t know what way to choose 🙂 blue has to go to C, but how it works now without it. If it goes to OB then do I need to put C and where is B then…

asking for your help maybe somebody has a response what to do with that! 

thanks! 

[Markjosephp]

Hello Val6,

Congratulations on your new Nest Learning Thermostat! It's always exciting to upgrade and enhance your home's technology. I understand that dealing with thermostat wiring can sometimes be a bit confusing, but you're in the right place for assistance.

Regarding your O (blue) wire and the absence of a C wire, you have a few options. The O wire is typically associated with the reversing valve in heat pump systems. If you're unsure about connecting it to the C terminal, you might want to share with us a photo of your old thermostat and Nest Thermostat's wiring setup. This would help us determine the wiring configuration.

Keep me posted.

Best,
Mark