Could be because part of the boiler's water jacket would not be cooled
by the water heater. You need to know what is actually heating the
domestic water

Whatever that is, if it can simply and safely be capped,
end-of-problem. True, the boiler will not be "same" as before. Any
competent plumber can give you the story as to your options with the
boiler. Oil company people would hopefully be knowledgeable about the
oil burner.

J