Fourteen full-scale fire tests were conducted to determine the optimal design parameters of water-based protection systems against large industrial oil cooker fires. All but one test fires were self-ignited by heating the oil inside a test pan above its auto-ignition temperature. Three test mock-ups that simulated industrial oil cookers were fabricated for the tests: 3.0 m long by 2.4 m wide, 6.1 m long by 2.4 m wide, and 12.2 m long by 2.4 m wide. Three types of water spray systems utilizing 13 mm orifice sprinklers, 13 mm orifice spray nozzles, and 6.4 mm orifice spray nozzles, respectively, were installed to discharge water with various discharge densities into burning oil inside a pan. Rapid fire suppression was achieved, although fire flare-up was very pronounced and the interaction between the fire and the water spray was very intense. Test results showed that the systems using the nominal 6.4 mm orifice spray nozzles placed in a double-row 0.76 m above the oil surface, discharging a 20 mm/min density, yielded the best performance. The tests also indicated that bronze sprinklers or bronze spray nozzles may not be strong enough to use against auto-ignition fires inside a cooker. None of the stainless steel sprinklers or spray nozzles showed any damage after they were exposed to several test fires.