I'd suggest Title 14 as a likely place to start looking. That's Portland Municipal code. As for others, they are scattered, but here's a couple...
Municipal Codes Web Library by LexisNexis
Portland's open carry ban applies(legally, that is), only to those without a CHL. However, they do still occasionally arrest for it. Were I to open carry there, or were my concealment less than perfect, I would expect to be put on my face on the concrete, and at the very least, lectured at length about the folly of my ways.
I am unaware of any place in the Oregon law where "concealed" is defined. As such, the default would be the dictionary, I'd expect, but better call the State Attorney's office or such, to be certain.
A good all around book to have is available through the
Oregon Firearms Federation. If I recall correctly, it's "Understanding Oregon's Gun Laws".
Oregon Firearms Federation | FAQ's. A Gun Owner's Guide In The Beaver State
This was a very handy guide for me when I moved to Oregon...
Here's the direct link for that book I mentioned.
Oregon Firearms Federation