Oakland has the 3rd highest violent crime rate per capita in the country:
http://www.businessinsider.com/most-dangerous-cities-in-amer...
And according to the link you posted, Oakland has less than half the population of San Francisco, but 37% more violent crime.
Yes, there are slightly more violent crimes in Oakland than San Francisco (and there's more property crime in San Frnacisco)[1], but on the whole they are pretty equivalent in terms of safety; especially if you stick to the 'nicer' areas of both.
[1] http://www.sfgate.com/crime/article/Crime-up-in-Oakland-much...