Kfahmi:

If you are at north field/Kaiser/Landmark at OAK walk 2-3 minutes to where the rental car buses drop off/pickup. Its one bus system, no Hertz or Avis buses. Buses to/from the main terminal go every five minutes or so. They do no care whether you rented a car or not. At least I never had a problem. There are even places you can park near to Kaiser, if you're sly, without going in their lot and can probably get away with it for a few days without paying anything, or getting a ticket/tow. I will not guarantee that, but again, its worked for me.