I think there are usually way too many unknown variables to safely diagnose something like this over the Internet.
Iâ€™m fairly new to this forum myself, so not really sure what the policy is on providing information for DIYâ€™ers.
However, assuming you do not have a known rodent problem, and all wiring is in good condition, I would probably start by investigating the heat pump. Are you sure it was not running? Does it also have electric heater strips?, I donâ€™t know why they would be needed with an oil-fired furnace.
If itâ€™s that old and routinely needs to be â€śresetâ€ť as you say, then there is already one defined problem that is going to get worse as time goes by.
With fuses, a locked compressor, bad start capacitor or potential relay could cause a high inrush/LRA which could possibly cause a main fuse to open before or at the same time as the time delay fuse clears in the heat pump disconnect, if there even is one, or the branch circuit. Maybe you should call an HVAC pro to have the unit checked out. Itâ€™s also possible that the problem had entirely nothing to do with the heat pump.
Quirky things do happen.
Iâ€™ve seen a problem with a single 20 Amp branch circuit trip a 200 Amp SQD QO main breaker before.
Sometimes in older homes, a problem with an electric range would blow one of the 60 or 100 Amp main fuses with or without blowing the 40 Amp branch circuit fuses.
That is why I say, there are usually too many variables to safely and accurately diagnose something like this over the Internet.
Maybe someone else here will have some other ideas. Good luck with it.