What brand and model thermostat do you currently have? The O and B wires are for the heat pump reversing valve and heating/cooling mode. The B wire is not a C (common) connection I am not sure if a single O/B connection will work with your setup. It might but you would have to get information on the the control board to verify the heat pump will work properly in cooling and heating modes.

I suggest you not install the Nest thermostat without a C wire connection. There is a chance it might work, or work for a short time until the internal battery dies.

Thanks, that's what I'm reading and leaning towards finding something else.

Currently have a very old American Standard thermostat.

How would you get info on the control board? Would this be in the manual for the air handler?

If you know the model of the air handler you may be able to find the installation documentation. In general Wifi thermostats need a C wire in order to provide the required power. If you can’t run an additional wire, the G wire could be used but you have to move it to the C connection at the control board. There are external devices you can connect to the thermostat to provide the required power source.

The Honeywell Home RTH8560D programmable (but not wi-fi) thermostat can run on batteries and without a c-wire.

The installation guide for the air handler should show and tell you about the connections. The user guide may also. In my case, I took the cover off the air handler and looked at where the thermostat wires were connected. I was surprised that one wire was connected to C (and ground).