The boiler I have is a very old United States Radiator Corporation cast iron boiler from the 50s
Most of the older boilers have an access panels that unbolt so you can replace the fire brick and blow out the heating chamber with compressed air and a hepafilter vacuum. The metal jacket might have panels screwed to the sides and top to indicate where the cast iron or combustion access is. If you remove any parts, make sure you seal them up to avoid air leaks. This is a tricky job and if any parts break, you will have a hard time finding them
your best bet is to call the oil company and have them come out and clean it for you.
Search oil boilers on internet, or get a Time-Life book on it or something. Too involved to explain step by step here. If you are not confident with it, call the furnace repair company.
us radiator has 2 doors in the front shut the main swich first. open the doors and using a boiler brush brush down the internal flue passages. while youre doing this vacuum out the soot and scale . thru the bottom bbor do the same process. vac the loose stuff which fell into the fire box. and thats the usuall process for cleaning tne boiler. you can also disconnect the smoke pipe and brush it and vac that too.
