Which of the following locations is it illegal to carry a firearm in Oklahoma?

Carrying a firearm in Oklahoma is indeed illegal on private property where a 'No Guns' sign is displayed. Oklahoma law allows private property owners to prohibit firearms on their premises. When a 'No Guns' sign is posted, it serves as clear notice that firearms are not permitted, and entering that property while armed can lead to legal consequences, including possible trespass charges.

In contrast, the other locations mentioned may not have such restrictions. For example, carrying a firearm is generally allowed in restaurants, state parks, and on public sidewalks, assuming the individual complies with any additional regulations or conditions set forth by local laws. This makes the restriction on private property with a 'No Guns' sign a unique legal stipulation that merits attention.