In California you may "open carry" anywhere you can legally discharge the weapon. Stated examples in the Penal Code are when hunting, fishing or at the range. You may not carry a loaded weapon ANYWHERE in a prohibited zone with out a exemption to the Loaded weapon law. That is any place you may not legally discharge that firearm.
For carrying a weapon in public, California has two laws. The first has to do with the carrying the weapon itself. You may carry concealed or on your person a weapon if you are going fishing, hunting, the range or seveal other destinations. THE CLINCHER IS IT MUST UNLOADED AND YOU MUST NOT HAVE ANY AMMO ON YOUR PERSON.
The second law makes it against the law to posses a loaded firearm in any public place. That is any place you may not legally discharge that firearm.
I live in a very gun friendly part of California, and I have a CCW, but I would be inviting trouble walking around town with a open carry. Most police do not know the law, and most time they will detain/arrest first and ask question later.