The blaze was first reported around 4 a.m. along Bell Ridge, which is about five miles from Rancho Santa Margarita.
OCFA said 200 firefighters were battling the flames with the help of seven air tankers and 11 helicopters.
One firefighter sustained an injury while battling the blaze, but the firefighter's exact condition was not immediately known.
No other injuries were reported.
The flames were spreading slowly on steep terrain, amid low temperatures and good humidity, the fire authority said.
No structures were immediately threatened, nor were any evacuations expected to be ordered, according to fire officials.
Additional strike teams were requested in the fire zone, as firefighters planned to attack the front end of the blaze from the air.