OA length is the length of the bullet from the base to the very end of the bullet.
Since all bullets have different ogives, bullets with the same OAL but different bullets will seat differently and be closer to or further away from the rifling.
This can and will affect accuracy.
The bullet ogive is the curve of the bullet. This length is different and is where the radius of the bullet curve goes from the flat to the beggining of the curve.
The base of the bullet is parralell. Where it starts to curve is the "ogive".
Clear as mud?