You want the Honeywell RTH8500D thermostat to control heat from the OWB and also run the heatpump without any additional controls? If so, I am not sure if that is possible. It looks like it is a 2 stage thermostat but doesn't appear to have the 2nd stage run a heat pump.
Do you just need to run the fan for the OWB heat or do you also need to control something else. You are probably best off using a strap on aquastat to keep the heat pump from running when the OWB lines are hot. The thermostat is most likely already energizing the G wire when the heatpump is calling for heat so you would just need to run the Y wire through the aquastat to break the connection when the pipe is hot. If you have an O connection for the reversing valve then you should be good with that. If you have a B connection then you may want to get a 2 pole aquastat and also break that wire too. If you don't, it will just cycle the changeover valve every time the thermostat calls for heat.