I thought I read somewhere that O.K. were the initials of a guy in a car factory who did the final checks of some parts or something, and his signal to say that the parts were fine and could continue in the production was- because it was too loud in the factory to shout- to hold up his hand in the O.K. shape. i.e. the car has been checked by O.K.
It means "fitting so well that the surfaces are just touching, so don't do anything else".