Gun laws are different for every state. I am only talking about Ohio in this post, but it's easy to find the open carry and concealed carry laws online. Keep that in mind, writers. Your character may be breaking the law if you don't know these laws. Heck, you may break the law if you don't know your gun laws.
Open Carry Laws
The situation for your characters may vary from a well-read officer who just nods to your character and watches, to a "green" officer (a rookie, I mean) who's nervous and arrests you for not having a license. Keep your setting in mind when you write scenes involving open carry, especially local law enforcement habits and experience. An experienced officer may be less jumpy than a rookie who just got a badge, but it depends on your characters and their personality. Your open-carrying character may get themselves killed by being upset loudly and resisting arrest. Again, depends on the character design.
Concealed Carry Laws
Concealed carry means it is concealed on your person or in your vehicle. Open carry doesn't apply to a gun in a vehicle. In general, watch signs on restaurants and know where you can legally carry. Someone can still ask your characters who carry to leave a restaurant, and legally they have to leave. Also, you can't legally drink and carry a gun, at least not in modern times. We aren't the old west anymore, so don't try it. However, you can get your character arrested for doing this if you want to.
In Ohio, you have to be 21 to get a license. You also have to have 8 hours of firearm training and no trouble with the law. Rule of thumb, if your character is a criminal with a record, they can't be licensed without a separate "clean" identity. In other words, unless they lied they won't get licensed for a concealed carry, and even if they lied they may be busted by a background check. You can't be convicted of anything or have a license from another state suspended to get an Ohio license. Another thing to remember about an Ohio license is that the license is accepted in 22 other states under reciprocity agreements. Your license takes 45 days to process and it comes from your county sheriff.
Where you can and can't carry
There are places you can't carry guns, even with a license. Rule of thumb, if you are in a building with law enforcement and courthouses you can't carry, as well as schools, mental health institutions, secure airport areas, and universities. Colleges make their own decisions on this, but I would say they still would make you leave it in your car. In your car, your gun is legal. Most colleges are probably not into students carrying, especially with school shootings that have happened on public campuses.
Where can your character legally carry? State parks, state forests, rest areas, non-secure airport areas, in your car, in certain daycare centers (unless a sign is up), private aircrafts, and anywhere else not listed. Double check my facts on this, just to be sure. If I'm wrong about anything, comment. I want to know if I'm wrong, for the sake of my writing.
Now you know
Now that you know what's legal, go get your characters arrested or go get your characters licensed and legal. That's your call, fellow writers. You can even get them killed for resisting arrest. Did I get anything wrong? Comment. I want to hear from you; what do you want researched? What are you writing about? Comment below and let me know!