Well, the closest you'll find to the OC is the Santa Ana Mountains, not much forest up there but there are forest service roads. A small OHV area can be found near Wildomar. The Santa Ana Mountains are nice for a cruise, but nothing challenging. You can drive up from the Orange County side and over and down east side when the roads are open.
Here's a link to the forest service page for the Cleveland National Forest service roads
Go further inland to the San Jacinto Mountains and San Bernardino Mountains and you'll find even more roads to explore within the San Bernardino National Forest
. Pleasant pine forests, more miles to explore (some roads are maintained by local 4WD clubs and provide a little more of a challenge), and maybe some stream crossings depending on the season.
Unfortunately, here in SoCal you don't have too many open OHV areas where you are free to roam. Ocotillo Wells out near the Salton Sea, and Johnson Valley near 29 Palms are two areas that I know of where you can roam openly. Plenty of people go out these places to camp and explore.
Forest service roads here are subject to closure depending on fire hazard conditions.