good source is
http://oregonfirearms.org/faq/
My understanding (guaranteed to be worth every penny you are paying for it) is that failing to abide by a "No firearms" sign constitutes trespass. (Aside from federal buildings and courthouses.) You will probably be asked to leave before being cited. Leave if you're carrying. You'll probably be OK.
One other thing: if you believe in gun rights, why not believe in property rights, and the right of a property owner to decide how visitors to his property behave?