The tick is certainly a species of Dermacentor - note the ornamented scutum. Dermacentor andersoni does not occur in Orange Co. to my knowledge, so it's probably the local Dermacentor occidentalis. - Barry

That's a big tick. I'd say 14mm for size...

Here's a link to the Info page for ticks.

Ticks can swell up like a balloon in order to store as much blood as physically possible. You can guess its normal size by looking at the head and legs. This one would have eventually dropped off and crawled off somewhere to lay eggs.

Although ticks on dogs aren't that uncommon an occurrence, it wouldn't hurt to mention it to your vet, since ticks can carry some dangerous diseases.

Looks like a bloated deer tick or some other species of tick.