Well, I called Portland PD's information line. The conversation went something like this (summarizing):
"Hi, I'd just like to get some information on the legality of Open Carry. If you're not federally prohibited from owning a firearm then you're legally allowed to carry it, correct?"
"That's right, but with that sort of thing it's also a matter of how you do it. If you're just carrying and wandering around, y'know... like if a woman is at a park with her kids and sees someone wandering around with a gun on their hip, they're going to feel intimidated. If you put your hand on your gun in a way that makes someone think you're going to draw on them that's harassing or intimidating and against the law."
"Right."
"And people can feel intimidated by anything, and it's really just a matter of whether they feel intimidated or not, so you can't just wander around..."
"Well there are specific laws regarding brandishing so that's not exactly accurate. Whether someone feels uncomfortable isn't really the same thing..."
"It is the same thing. If you're wandering around with a gun then people are going to be uncomfortable and that's harassment."
"There are specific laws about harassment and that's not really true"
"Well if you already knew then why are you calling me?"
"That's not what I called to ask you"
"Alright, you've got another question then bro?"
"...yeah. I'm just wondering, if you have a record but it doesn't prohibit you from being able to own a firearm, then you're still legally allowed to carry, yes?"
"Well it depends entirely on the record. Not all records are going to allow you to be able to own a firearm. If you have a restraining order or a felony..."
"Right, I know that, that's why I qualified my question."
"You know what dude, if you're going to be a smart ass then this conversation is done. [Some sort of moral grandstanding that I didn't hear because I hung up at this point]"
So I'm kind of unsure of the situation at present. The only straight answer I've gotten to my question here or anywhere was Nightmare's which was vague, unsupported with any evidence, and grammatically awkward. So really I have no answer to this at all, and now it's clear that Portland PD are unprofessional and biased about this subject.
I guess it's worth noting even if it subjects me to some harassment on these forums that I'm MtF transsexual as well and not exactly very good looking, so the whole "getting harassed by cops" thing is liable to be compounded by that, let alone the "making random soccer moms upset so that they call the cops" thing. What I would really like is a cite to precedent or statute or some other professional opinion, addressing the exact question I laid out in the OP. At the very least, an entire paragraph worth of answer on Nightmare's assertion (that if a record doesn't prohibit you from owning a firearm, but the actions that lead to that record would have prohibited you from owning a firearm had they occurred in the state of carry, you're not allowed to carry). I feel like all I'm getting right now is jerked around.