Let me first address the elephant in the room. Yes: Oakland is dangerous. No: Oakland - at least most of the places one would go - is not any more dangerous than the rest of the Bay Area.
Property crime is endemic to the whole urban area. So if you're going to take it personally when someone steals your car or your kid's tricycle (really, thieves?
), I guess move out to the suburbs. Rich areas get targeted for property crime as much ad poor areas.
Violent crime, on the other hand, is less widespread. As a matter of fact, as scary as the murder numbers are, what's scarier is that they're almost entirely in 5-6 neighborhoods. This map is a little dated (the data is from one month in 2008), but shows the general pattern of violent crime - murders, robberies, assault, and rapes. Each violent crime is a red dot, and I've added green and orange circles to sort of highlight that most neighborhoods have little violent crime.
Actually, since 2008, Uptown has been radically gentrified, carving out a little violence-free bubble. Downtown west of Broadway has also been gentrified.
This pattern basically holds for most of San Francisco, Berkeley, and Oakland. Whether you're visiting or looking to live in the area, your exact neighborhood is going to matter a lot more than the name of the city.
The bottom line is that you should take Standard Big City Precautions when you're in the area - whether you're in Union Square or at UC Berkeley or Downtown Oakland - but you don't need to fear anything above Standard Big City Precautions.
Unless, of course, you wander into red-dotted neighborhoods looking for drugs, prostitutes, or gang members. (Few large-scale gangs exist any more, most violence is perpetrated by micro-gangs with small turf sizes.)