This isn't so hard after all
Old bimetal thermostats used extra contacts on the mode select (heat/off/cool/e-heat) switch to determine whether O or B was energized, so as a result, your old thermostat provided both an O and a B terminal, in order to be compatible with all heat pump systems, as some require an O wire while others require a B wire. However, there are very few systems (certain residential zone boards are the only setups I know of) that require both O and B to be present; all heat pumps and many other systems that use O/B require only one or the other.
In your case, your O wire is connected to your heat pump reversing valve (the orange wire in the thermostat cable is connected to the orange wire going off to the heat pump), while the black wire in the thermostat cable is unconnected at the furnace end of the system, apparently. As a result, the orange wire goes to the O/B terminal in your new thermostat, while the black wire can safely be left unhooked, since it goes nowhere anyway. Also, the white wire that was connected to E on the old thermostat needs to go to AUX on your new thermostat, due to the way it handles auxiliary/emergency heat. The rest of the wires go to the obvious terminals: red to R, blue to C, yellow to Y/Y1, and green to G.