Federal law says you have to be 18 to poses a handgun, in Missouri I worked in a gun shop and sold several handguns to a father who then gifted his <21 year old child. It depends on what OK law says as the minimum age to poses.
Also as per ATF seminar I went to; it matters not who pays for the firearm, they just want the ultimate possessor to fill out the 4473 and it is not a straw purchase (as long as the person filling out the 4473 is over 21).