All campsites in LPNF are first-come, first-serve, but even on the weekends (barring that it's not a holiday weekend), there are plenty of awesome backcountry campsites. Oak Flat has several established campsites, some are closer to Sespe River than others, but the one we recommend is a spacious group campsite further away from the river and closer to the mountain side.
Why? Because it has two picnic tables, plenty of shade, space for multiple tents, wooden benches, and a giant fire pit. It is an equestrian campsite so you have to deal with a bit of horse poo on the outskirts, but the picnic tables and benches are perfect for larger groups.
The name Sespe can be traced back to a Chumash Indian village, called Cepsey, Sek-pe or S'eqpe' ("Kneecap") in the Chumash language
Sespe Wilderness is also home to the Sespe Condor Sanctuary. It's not open to the public, but it was established to protect one of the most endangered animals in the world. What's a major threat to condors? Microtrash (i.e. bottle caps, broken glass).
California has the second largest number of wilderness areas in the country (Alaska has the most)