YesI mean the oil pipe.
they are usually 10mm and are easy to heat with a run of warm water.
From the boiler to the house, you will have 2 pipes (flow and return), 1inch, and given the age of the house, the pipe is likely to be gun barrel.
Its possible for either pipe or both to freeze. You can (for the moment) assume that the pipes are at a sufficent depth under ground not to freeze, However Look at the pipe as it enters the ground at the boiler, it could have frozen here. Then do the same where the pipe enters the house.
What I would do - is get 2 pump wrenches and loosen the compression fittings near the boiler. (Dont take them off completely - just loosen) see what the pressure is like. It should start spitting out at pressure. If the pipes are frozen from the boiler to house or vice a versa are frozen, you will have very little pressure.
People generally are afraid of water and plumbing, but if you think about it logicly and DONT completely open the fittings you should be ok.
I have had to do this in the past and found the pipes blocked where the pipe entered the house.