Ohio law is as follows:
2923.20 Unlawful transaction in weapons.
(A) No person shall:
(1) Recklessly sell, lend, give, or furnish any firearm to any person prohibited by section 2923.13 or 2923.15 of the Revised Code from acquiring or using any firearm, or recklessly sell, lend, give, or furnish any dangerous ordnance to any person prohibited by section 2923.13, 2923.15, or 2923.17 of the Revised Code from acquiring or using any dangerous ordnance;
(2) Possess any firearm or dangerous ordnance with purpose to dispose of it in violation of division (A) of this section;
(3) Manufacture, possess for sale, sell, or furnish to any person other than a law enforcement agency for authorized use in police work, any brass knuckles, cestus, billy, blackjack, sandbag, switchblade knife, springblade knife, gravity knife, or similar weapon;
(4) When transferring any dangerous ordnance to another, negligently fail to require the transferee to exhibit such identification, license, or permit showing him to be authorized to acquire dangerous ordnance pursuant to section 2923.17 of the Revised Code, or negligently fail to take a complete record of the transaction and forthwith forward a copy of that record to the sheriff of the county or safety director or police chief of the municipality where the transaction takes place;
(5) Knowingly fail to report to law enforcement authorities forthwith the loss or theft of any firearm or dangerous ordnance in the person's possession or under the person's control.
(B) Whoever violates this section is guilty of unlawful transactions in weapons. A violation of division (A)(1) or (2) of this section is a felony of the fourth degree. A violation of division (A)(3) or (4) of this section is a misdemeanor of the second degree. A violation of division (A)(5) of this section is a misdemeanor of the fourth degree.
You do see them for sale in off brand convenience stores around the state as well in flea markets and odd small businesses here in Ohio. I don't think even law enforcement here cares that much unless they target you or if you are using them in a fight. However, if you are found carrying therm and they want to prosecute, you are screwed here. You can collect brass knuckles and/or keep them in your home. Technically though transporting them if caught is a felony offense. Many novelty stores and online businesses sell them as "paper weights" made from stainless steel, brass and now in plastic. The moment you are caught with them on your person outside the home ( even as a "decorative" belt buckle, it is possession of a dangerous weapon ( felony charge ) same as carrying an unloaded gun. Possessing a concealed carry permit does not allow you to carry brass knuckles. That is for firearms only.