There is no "official" sign in Ohio, only a recommendation of what the sign should or could look like. As long as it is conspicuous enough for you to see it without hunting around and it's intent to have you disarmed are clear, it's legal. But as soon as you know they are posted no matter how inconspicuous, or their intentions are made clear to you, you are required to leave immediately. So yes, that sign is legal and even if it is hidden away somewhere, because you have taken a picture of it have shown you know they don't want you to carry, you are legally bound to adhere to that sign.