What model pump do you have and is it pushing towards the tank?
An expansion tank will do nothing on an open system. A large pump before the tank and restrictions after the tank could cause the pressure to bulge the tank.
I was not able to find pressure ratings on oil tanks other than to not test above 5 psi.
A large pump could get the pressure to near 20 psi if the system gets blocked completely after the tank. (close valve, plugged hx, etc)
If you have a small pump you are probably fine. If it is a larger pump, I would think about replacing it with two smaller pumps, one before and one after the tank.
Or just get rid of the tank if it isn't necessary.