A FIRE OCCURRED ON THE OIL COOLER FOR #4 TURBINEGENERATOR IN THE MONO POWER HOUSE. THE FIRE OCCURRED WHEN AN EMPLOYEE LOOSENED AN ""O"" RING TO REPLACE THE H2O TUBES. THE SYSTEM WAS UNDER 25 PSI PRESSURE CAUSING OIL TO SPRAY ON A 600# STEAM LINE WITH SURFACE TEMPERATURE OF APPROXIMATELY 750F. THIS CAUSED THE OIL TO IGNITE STARTING THE FIRE.