I can see some merit to the argument that the one that is open carry might be the first target, but I think in reality that most would be robbers and assaulters would think twice about messing with someone with a gun strapped on their side.
Not necessarily. It depends on the totality of the circumstances, as in everything else in life.
Which is why it's of prime importance, regardless of how you carry (indeed, even if you DON'T carry) that one is alert to one's surroundings.
For example, a person who is bent upon a violent attack will gauge the measure of his violence based on his perceived threat and opportunity. If you obviously have a weapon, then his chosen means of attack will likely be of a more lethal variety.
Put yourself in the shoes of an attacker committed to the act: Your chosen victim is obviously armed, walking through a secluded park area in the early evening. Nobody is around. Instead of risking a direct confrontation, you lie in wait and shoot your victim from where he can't see you. Take his wallet and run. Time spent? Seconds. Risk to self? Minimal.
Obviously, this is a chosen scenario to illustrate an example. And, like any other example, does not reflect the totality of all circumstances.
Which is the point...one can no more say that OC will deter any given attacker under any given circumstance than another.
OC or CC as one sees fit. But DON'T overgeneralize to the point where you're essentially lying to yourself and thereby placing yourself on unsound ground.
The important aspect of either OC or CC is the "carry" part. Exercise sound logic and reasoning either way you choose, and don't put on blinders about it.