To test an oil furnace transformer, first ensure safety by disconnecting power to the unit. Then, use a multimeter to check the primary and secondary voltages; this confirms if the transformer is functioning correctly. Inspect for signs of damage, such as burnt or corroded terminals and insulation. Next, measure the resistance of the windings to identify any potential shorts or open circuits. Finally, verify the output voltage under load conditions to ensure it meets the specifications required for your system. Regular testing ensures optimal performance and longevity of your oil furnace transformer.