Radirators can be very trickey to get working.
Starting by purchasing your waste oil heater with the copper coil fitted. Or a log fire with a wetback fitted which is converted to waste oil.
You need to plumb it up using an Ajax Valve teed on the cold inlet side of the coil, which is the bottom of the coil.
The Ajax valve connects to the town supply water, to keep the closed loop full of water all the time.This can be as easy as connecting the garden hose to the Ajax Valve.
An open vent is needed, extending at least 2 metres above your radiator, this needs to be teed into the hot side of the coil, or the top of the coil. Ideally vented out on your roof so hot water and steam can exit to the stormwater. The vent is just an open 15mm copper pipe.
Fill the loop up with the town supply water. Turn the Ajax pressure screw until the water exits out the top of the open vent, then turn it back one turn. Stopping the water from flowing out the vent. This is called the water head.
Next is bleeding the air out of your radiator, until only water exit the bleeding valve at the top end of the radiator.
Light the heater, The heater needs to be running at 400C to heat water around 85-100 degrees C.
If your using a pump turn it on to circulate the water, or you will hear a few bangs as the hot water tries to move the stubborn cold water, if using thermosyphon system. This is the noise of the water being pushed up the pipes, it will stop once the water warms up and starts moving.
If the bangs carry on, then there will be an airlock in the radiator so bleed the radiator more until only water exits.
If nothing comes out the bleed valve, then the Ajax Valve needs to be turn up further until it does.
If you plumb up a radaitor without an Ajax valve or an open vent you are creating a bomb waiting to go off, yes it will cost abit more but then you know it's been done right.
The other way is by using an Expansion vessel.
If you have a vented water heater, there is no need to have any form of expansion vessel, because the expansion is taken care of by being vented to the atmosphere through the special vented taps. So you only need to consider an expansion vessel if you have an unvented water heater.
The waste oil heater, I think when it boils water in it's copper coil it creates air and your alway going to have that issue. So instead of manually having to unsrew the airvent nipple I replaced it with a solar airvent to remove that air automatically. Should have used a brass 90 degree bend rather than the galvanized in the photo. But it works.