My thoughts are most thieves are nervous while committing a crime. Theoretically if I were to rob a place I'd be nervous as hell and looking around quickly trying to look out for a uniform. If you are OC'ing and are relaxed, most people will not even pay attention to you, let alone look towards your waist. Recently a buddy brought a new g/f over to my house for dinner & a movie. She was nervous and apparently never noticed the gun on my hip. My buddy mentioned to her how nice it was she didn't make a deal out of my carrying (she's from Illinois), she replied it was because she didn't notice I was carrying.
If a robber is smart enough to look for firearms, they are probably smart enough to not even bothering to rob that place.