where do drop off the rental card in Oakland airport?

There is a rental car return driveway at the entrance to the rental car facility it services all rental companies just follow the road until you see your rental company's name then enter the corresponding driveway.

At the rental car facility on 7600 Earhart Rd. Shuttle buses available to and from the airport

All the car rentals are in one location and well marked even with the construction.

Follow the signs for rental car returns. They are next to the airport. All rental car companies are in one area together. Drive along the path when you get there and see the signs for the specific rental car company.