Zoned oil furnace and AC thermostat question
You should check the wiring in the furnace. There are no standards for thermostat wire colors, so you can't always trust the color of the wires.  Seeing how the wires connect to the furnace, is a sure fire way to figure out what's what. 

The O (cool mode) and B (heat mode) terminals are usually used for a reversing valve, in heat pump systems.  I'm not familiar with oil systems, so I can't say for sure what they would be used for. 

Some thermostats have separate O and B terminals, like this.




Others have an O/B terminal, and use a jumper to select which should be used.