Someone I know carries a Vz. 52 pistol OWB. It hangs out in the open almost all of the time. After several days of wet weather, the pistol was rusted. Oops. It looked horrific (sorry – no pictures). Rust on the barrel, between the recoil spring and barrel especially, and rust on the outside where it contacted the holster. Even some of the cartridges had rust on them from the magazines. After taking it down, almost to the last pin and the last screw, it cleaned up very well. Nothing serious this time. I’m sure the piece would have functioned, though metal oxides can be extremely abrasive. It could get really bad if left in the holster for a longer time.
Be careful out there if you OC. My pistol is almost always covered at least by a shirt and I’ve never seen signs of rust on it, so I’ve never thought much about it. I have left a Winchester carbine in the vehicle for weeks at a time, and in very cold weather condensation can get between the metal and the gun case interior, causing rust at all the contact points. So you have to take extra care. The Parkerizing on the Colt seems to handle it much better, and the annodizing of course is already a hard metal oxide, but you want to be checking these things.