I would say that you have to have the knife visible at all times. There is no test for what is "open carry" as far as I know, but if an interested observer can easily tell you're wearing a knife by looking at you, that should probably satisfy the rule.
So if it's covered by a shirt or jacket, it's not open carry. Definitely do not carry a concealed fixed blade in California! Most of the time you will *not* just be let go and lose your knife. The penalties for carrying a concealed, loaded gun are lower (two misdemeanors as opposed to a potential felony). And you *can* legally carry a concealed, closed, locking, thumb-opening folder of any length by state law (some local laws restrict blade length, especially if the folder is not completely concealed, but most cops are not aware of them).
For your reference, here is the California law prohibiting concealed carry of a fixed blade:
California Penal Code Section 12020. (a) Any person in this state who does any of the following is punishable by imprisonment in a county jail not exceeding one year or in the state prison:
...
(4) Carries concealed upon his or her person any dirk or dagger.
...
(c)(24) As used in this section, a "dirk" or "dagger" means a knife
or other instrument with or without a handguard that is capable of
ready use as a stabbing weapon that may inflict great bodily injury
or death. A nonlocking folding knife, a folding knife that is not
prohibited by Section 653k, or a pocketknife is capable of ready use
as a stabbing weapon that may inflict great bodily injury or death
only if the blade of the knife is exposed and locked into position.
...
(d) Knives carried in sheaths which are worn openly suspended from
the waist of the wearer are not concealed within the meaning of this
section.
