@ Dakaham
Here's a link to the ORS regarding firearms regulations in Oregon. I would suggest reading 166.170, 166.73, 166.250, 166.260 at a minimum.
http://www.leg.state.or.us/ors/166.html
The ONLY part of the Salem code that is different from ORS is tha they have chosen under 166.173 to prohibit loaded carry by non CHL holders. Since Oregon is a Shall Issue state, most of us have our CHL's for the added benefit of being able to ignore such local regulations.
As Tedde stated, Oregon State Fair is a public venue.
As it is operated by a politcal subdivision of the state, they can not restrict firearms like a private venue could.
As for conact by an LEO, since it is not unlawful to carry and Salem regulations do not grant authority to do a "load condition check" they would have no RAS to detain an individual. Any contact would therfore have to be under "consensual" conditions and ID is not required, nor is consenting to the contact.
It's really quite simple. This is OREGON, OPEN CARRY IS LEGAL.