However, don't confuse a private property owner's right to ban firearms on their property with making it illegal to carry a firearm on their property. Two entirely different issues. In Oklahoma, a sign prohibiting firearms on private property such as Wal Mart only expresses the desire of the property owner to not have handguns on their property. It does not make it illegal to carry a gun on their property. If they ask you to leave because of your gun, and you do not leave, you will be guilty of trespassing whether or not they have a "no guns" sign posted anywhere, but not necessarily illegally carrying a firearm.
Now, in Texas, if the sign complies with 30.06, to carry past that sign, in most cases, would be illegally carrying a firearm.