Need Base access since it is on Camp Pendleton. Need California State license and also a base fishing permit which you can find here:
https://pendleton.isportsman.net/
Lake Oneill has Large and Small Mouth bass, Channel Catfish, Common Carp, Black Crappie and Bluegill. ****Bass are "catch and release" ONLY!
Most of the Lake has Tule growing along the banks so you have to search for spots for bank fishing although there are a few docks around the lake for fishing. There's also a boat ramp but check the base website for regulations. The Lake is small and I mostly see people fishing from fishing kayaks.
The Lake is a man-made reservoir so there is no swimming or body contact allowed with the water and no pets allowed in the water.