Continuing, be sure to go to:

http://bart.gov/guide/airport/oak.asp

to read about the AirBART shuttle bus. I have used it at all hours and have never had any problems with it. The station and the airport are both well patrolled and the AirBART bus is nice and clean and non-stop between those two points. PLEASE NOTE: AirBART bus has a unique fare collection system, so read all about it, and make sure your ticket has just enough extra $$$ on it to hand to the driver.

Now there is also another bus run by AC Transit that is cheaper (or used to be) and makes stops between the station and the airport. This bus comes from downtown Oakland via MacArthur, IIRC. This bus I would avoid in your case, as the fare savings is only slight, and it is not as nice as the AirBART bus nor does it have luggage racks of course, but I have taken it a couple of times in the daytime when it arrived before the AirBART.