Authorities say the blaze is not threatening any homes, but crews were on hand in case the winds shift. The fire is about 50 percent contained and has burned about 400 acres.
The fire is near the community of Olinda Ranch, which is right next to Olinda Village.
The fire broke out a half-mile west of the neighborhood of tract houses and mobile homes and north of Carbon Canyon Road at about 11:15 a.m., Brea- Fullerton Fire Chief Wolfgang Knabe said.
Just before 1 p.m. the blaze was about 15 to 20 acres, but it had expanded to 100 acres by 3 p.m.
Olinda Ranch resident Steve Grushen said he came home early from work after hearing about the nearby fire.
"I started having to think about what to pack and do I get the cat out and how long to stay around," he said.
Roads leading to Olinda Village were closed. Authorities do not expect to have the fire contained until at least Friday.
About 200 firefighters have responded to the fire. No evacuations have been ordered.