person may buy or own under state law

In Oklahoma (under state law), private sales of firearms are legal. However, knowingly selling a firearm to a person who cannot legally purchase or possess the firearm (such as a convicted felon, minor, or drug addict) is illegal, and punishable by up to 180 days in jail. Under state law, you must be 18 to buy a firearm of any type (handguns or long guns) from a private seller.
Prohibited Persons

Felons, mentally ill persons, drug addicts, and certain juvenile offenders are prohibited from buying or owning firearms. You must be 18 years of age to possess or own firearms, with a few exceptions for special circumstances. When purchasing a firearm from a dealer, you must complete a federal background check. This process (passing the background check) usually only takes a few minutes, but the government has up to three days to complete the background check. After three days, the dealer has the right to proceed or deny the transfer. There is no limit to the number of firearms that a person may buy or own under state law.
Minors and Firearms

Minors may own firearms that are given to them by their parents, relatives, or legal guardians. However, they may not generally possess firearms unsupervised until they are 18 years of age. There are exceptions to this rule, such as when a minor possessing a hunting license (and having passed a hunter's safety course) is hunting, possession of firearm(s) on certain types of private property, and possession of firearm(s) in one's own home.
NFA Firearms