There is a Marriott and a Courtyard right on top of the 12st Oakland BART station, but in the really big conventions they are sold as convention hotels as well.

I used to work in Chinatown/Downtown Oakland Jack London Area. Here is the deal in super short list

JLS : fantastic food. Very safe. Has a ferry ride to SF, but the schedules are weird. The walk to BART is safe, but it might be long to some people clocking in at about a mile.
Chinatown is awesome, super authentic, dirty as hell and graffitied, safe and next to BART.
Oldtown Oakland has the Marriotts. Weird people out, doing drugs and sleeping at the bus stop, but they don't bother you. Also very Safe. Oldtown also has some fantastic restruants and a Smart and Final grocery store.

The Walnut Creek Marriott is walkable to BART, its about 15 minutes if you take the side streets in an office park type setting.