I still have my propane fired boiler in the basement and have the Hardy plumbed into that system. I too have my propane boiler as a back-up. What I have found for my system is this, during the early and end of the normal heating seasons, my circulating pumps dont run continously bringing hot water into the house. That allows the water in the line from my boiler outside to the one inside to cool off. So when one of the pumps kicks in it will initially be bringing cooler water in than what my propane boiler's aquastat is set for and that boiler will fire to offset that water temperature difference. Once the water is hot coming in ( above 120 F )my propane boiler shuts down. During the colder months when my system is calling for hot water almost constantly and I have at least one pump circulating ( my house has 3 seperate heating zones/pumps ) my propane boiler never fires. I end using about 200 gals of propane a year.
You should have your oil fired boiler's aquastat set back to kick in at around 120F. If your Hardy is set to shut down at 180 like most are, you should be carrying about 160 F water into your house from the Hardy pretty steady and your oil boiler needs to be set below that or it will always be running too. I have a temp gauge on my propane boiler and I can montitor my incoming water temp and mine usually stays between 140 and 160 when my pumps keep circulating.