Sorry, maybe I spoke a bit too quickly. But, I believe that Oregon is a shall-issue state...no?
Anyway, if you're carrying a knife purely with SD purposes in mind...there's legal issues. Carrying a utility knife is a better idea, f.ex., a delica/endura would fit this bill well. However, without training, a knife is little better than empty handed, and definitely worse than a stick. I'd recommend taking the money you were going to spend on a knife and getting some SD training first.
A folding knife has the same disadvantages as a pistol - takes time to get it into play. Most of the time, muggings or whatnot won't happen with the "ambush out of the dark" - type move, rather, it's usually one person to distract, and another to surprise you when your guard is down. In that case, unless the knife is in your hand, you won't have time to get it out.
Therefore, the best defense against such incidents is simply staying alert and aware of your surroundings.
Note: I do have little experience in this, so take my words with huge grains of salt :-d