Oakland is like any other city. There are safe neighborhoods and dangerous neighborhoods. Some of the safer ones are: Temescal, Adam's Point, Grand Lake/Cleveland Heights, Piedmont & Rockridge (not technically Oakland).
OTOH, I live in Oakland and drive. I've never gone to anything in Oakland except Amazon locker pickup, Whole Foods (but prefer Berkeley places), and the 580/880 onramps. I step into my car in an underground garage and essentially don't get out until I'm in the civilized/Promised Land (Peninsula).
Berkeley is also maybe more fun as an intern, even if you work in SF, and has BART. I'm not sure what summer housing is like there -- I'd assume there are vacancies in places rented to students during the year over the summer.
Nitpicking: Piedmont is an enclave contained within Oakland (but independent), but Rockridge is part of Oakland. (it's also "Adams Point", not Adam's Point)