Actually, 166.173 and Portlands city ordinance are the controlling elements of law in the OP's example.
166.173 grants Portland the authority to regulate LOADED firearms in public places for those without a CHL. Since the OP does not have a CHL and 166.173 does not offer protection for licensed hunters and fishermen, he can be regulated by Portland city ordinance.
166.250 doesn't apply at all since the OP does not have a CHL but does have a hunting/fishing license and is ONLY asking about when he is on his way to/from hunting/fishing. This is found in 166.260(3) which states that "ORS 166.250 does not apply to or affect:" and "(3)(b) Licensed hunters or fishermen while engaged in hunting or fishing, or while going to or returning from a hunting or fishing expedition".
166.250 (4) makes a CHL an "affirmative defense" to a charge of violation of 166.250 but 166.260 (1) and (1)(h) make it quite clear that 166.250 "does not apply to or affect" CHL holders. The intent of 166.260(4) is merely to make it clear that a charge of violating 166.250, brought against a CHL holder will not withstand judicial scruitiny.
Now when we turn to Portlands city ordinance the wording is slightly different. It says that the hunting and fishing license (among other things) is an exception to the ordinance and constitutes an affirmative defense. Still, this is intended (though I'm sure they wish they didn't have to) that hunting and fishing licenses exempt the holder from the ordinance if they are going to/from hunting or fishing. The affirmative defense language does the same thing it does in 166.260(4)...to ensure that there is no question that charges will fail to pass judicial scruitiny.
Under the law, an arrest cannot be PROPERLY made unless there is a law, that applies to the individual being arrested, that has been violated. Since 166.250 does not apply to CHL holders or licensed hunters and fishermen going to/from hunting and fishing (per ORS 166.260 (1) and (3)), no PROPER arrest can be made under the premise of violating 166.250. The same is true of the Portland city ordinance under its provisions.
Now, in the real world, can the cops arrest you and haul you in (whether using a CHL or a hunting/fishing license)? Sure they can, they have the badges, the guns, all kinds of buddies, and the backing of the state to use force. In fact, they can arrest you because they just don't like the way you look, or whatever other reason they want to give. HOWEVER, it would be an abuse of their power, a violation of the citizens civil rights, and should result in charges being dismissed and civil suit being filed (and won) against the department and the officers as individuals.
Be sure to read my tag line....IANAL......but I am a Criminal Justice scholar