So from the reading of this law, as presented, one could be charged for having a pen or pencil, or anything else that "could be" used as a dangerous weapon. "Any other instrument?" Leaves it very open to interpretation.
As for: "while in or on a public building"
Not exactly true. I'm in Klamath Falls. Our courthouse is a public building. However, my CC is legal, by law. However they will not comply. Note: Refusal to comply with state law.
As for: "Unless you have a CHL your committing a Felony
There are public buildings, notably the Circuit Courts and the Oregon Supreme Court (for instance) that do not exempt CC holders. But these exemptions are few.
Both Public Building and Dangerous Weapon have legal definitions in the statue.
For Dangerous Weapons under ORS a pen or pencil or anything else that could be used would not be a Dangerous Weapon. That is a straw man argument and I'm willing to be you know it's BS. Any other instrument is not open to interpretation because Dangerous Weapon has a very clear legal definition.
From ORS 161.015: "Dangerous weapon" means any weapon, device, instrument, material or substance which under the circumstances in which it is used, attempted to be used or threatened to be used, is readily capable of causing death or serious physical injury.
Serious Physical Injury also has a legal definition. So yes, a pen could be a Dangerous Weapon under ORS at the point in which you go to stab me in the eye with it, but since using it to write on a note pad does not make it readily capable of causing any kind of injury at all, except maybe to my ego, it isn't a Dangerous Weapon and thus isn't going to run you afoul of the law. You can carry a bucket of acid from the delivery truck to the chem lab, but once you try to throw it onto someone it has the legal status of being a dangerous weapon and you're now guilty of a felony, but before that it was just acid.
I don't know anything about Klamath Counties refusal to comply, however Public Building also has a definition and Courts are not "Public Buildings" for the purpose of this statues. Courts are Courts. Yes I know that there is a public part of many buildings that we would consider "Public Buildings" but because Public Buildings has legal penalties attached when certain behavior happens at them, it has a definition that limits the scope of the ORS.
Section 2 of the relevant statue (166.370 Possession of firearm or dangerous weapon in public buildings or court facility) says you can't have a gun in a Court Facility, full stop. But wait, Court Facility has a definition and it includes only these kinds of courts: a courthouse or that portion of any other building occupied by a circuit court, the Court of Appeals, the Supreme Court or the Oregon Tax Court.
We finally get to Local Court Facility later in the statute which has a definition: "Local court facility" means the portion of a building in which a justice court, a municipal court, a probate court or a juvenile court conducts business, during the hours in which the court operates.
The statue then says: A firearm in a local court facility is guilty, upon conviction, of a Class C felony if, prior to the offense, the presiding judge of the local court facility entered an order prohibiting firearms in the area in which the court conducts business and during the hours in which the court operates.
Edit: Sorry, I forgot to address your other point, there are no "Public Buildings" where a CHL won't grant you the exemption in Oregon, but please note that is true because Courts are not "Public Buildings" under the law, they are something different and yes, the CHL does not grant you an exception there.
I don't know the situation you are referring to in Klamath, but a Local Court Judge does have the legal power to ban firearms in the areas listed above within a local court during the hours that the court operates.
Clear as mud?