The best thing to do is to take the front panel off of your furnace and find the wires that are running to the switch. You will need to get a test light and make sure when you flip the switch that the light goes off also. If it just stays offyou need a new switch.
Check wiring, turn off electrical power, disconnect the fan at the fan motor,set thermostat to call for heat, watch the burner. Reverse the prior steps.
The limit switch should automatically turn on the fan when you adjust the heat at the thermostat. To start, you should simply turn the heat up at the thermostat. If you notice that the fan does not turn on shortly, there may be a thermostat problem instead of a switch problem.
Oil furnaces are extremely dangerous to troubleshoot yourself. It's likely not the limit switch, the limit switch is probably preventing an explosion, sounds like it's overfiring. Call a service person.
