Sometimes both the O and B wires are connected at the thermostat but the wires are not actually attached to anything at the other end on the system control board, so it's worth checking that.
Strictly speaking Nest Thermostats are not directly compatible with systems that need both O and B wires. Those wires control when your heat pump cools or heats your home. It is possible to add a relay to resolve this, usually at a zone control panel, in order to make it compatible but for that, I'd recommend consulting professional help. I'll link below guidance on how to do that.
Get installation help or HVAC service from a professional - Google Nest Help
