I've seen this in Huckleberry too. Cap 5 to 30 cm in diameter, light to dark brown with faint rings, chalky stem, gills bleed latexy stuff, grows under oak. I'm guessing some kind of Lactarius, but I've yet to bust out Demystified. LOTS of these things out there. Like, lots.
Ok, after walking through the Lactarius key in DM, I think this might be the Golden Milk Cap (Lactarius alnicola). I didn't taste it for bitterness, but it seems to match all the other criteria, especially it's abundance under oak.
Observed near in Briones Regional Park, near Lafayette, CA.
Lactarius alnicola, commonly known as the golden milkcap, is a species of fungus in the Russulaceae family. The fruit bodies produced by the fungus are characterized by a sticky, vanilla-colored cap up to 20 cm (7.9 in) wide with a mixture of yellow tones arranged in faint concentric bands. The stem is up to 5 cm (2.0 in) long and has yellow-brown spots. When it is cut or injured, the mushroom oozes a white latex, which has an intensely peppery...