He's stating, that if there was ONLY OC, and there was NO CC, then the bad guys would know who carried the guns.
Therefore, they wouldn't be worried about attacking people not showing guns.
Therefore, CC will cause some worry, that they will run into someone with a gun.
Based on the follow-up message, I believe that "If only OC was legal. . ." was intended to convey "if open carry exclusively were legal and concealed carry were illegal."
As written, most of us would interpret as you did and I did.
A better phrasing using the same words would be "I open carry only were legal."
Now I get it. Thanks fellas!