Both sides of the debate have a dearth of usable information so it really does come down to "this is how I feel."
I'm sorry. I didn't elaborate.
Allow me to, please.
Do I think that a potential robber SEEING a person with a gun could be a deterrent? Definitely.
Do I think that he will just shoot someone carrying openly and proceed with the robbery that he had planned? No, I don't.
Do I think that would-be robbers see a uniformed cop or guard and put off their crime for another place and day? There's no question. That could be as much about the uniform as the gun, but either way, I'm certain it's a deterrent.
Do I think that an armed robber walking into a crowded fast food joint, nervous and maybe hopped up on speed will see my gun in the crowd BEFORE he pulls out his and tries to rob the place?
(This scenario is from a news report in American Rifleman's Armed Citizen section, I did not invent it. Armed robbers do try to rob Burger King -- and they're the STUPIDEST robbers, too, no doubt.)
No, I don't think he'll necessarily see the gun on my hip BEFORE he starts the robbery.
And there's the rub. If he doesn't see it early on, it can't possibly be a deterrent.
If he sees the gun once the robbery is in progress, a nervous robber might well open fire at me, and in a crowd of innocent people, before I ever get a chance to draw it. THAT is when I'd be a target. Question that if you want, but that's the scenario where I'd become a target in my opinion.
If my gun is hidden in my pocket holster, I can decide if and when I'd shoot, and draw without attracting undue attention. This could well save lives: mine and the innocent bystanders. I want the tactical advantage to be MINE, not the robbers.
So, in crowded places, I am far more likely to conceal.
Elsewhere, I do think my gun would be a deterrent more than a danger. Maybe I'm wrong about that, too.
My point is just this: OC might well be a deterrent, but ONLY if a would-be criminal sees the gun BEFORE he decides to commit the crime.