Actually, it's kind of an interesting story here.
OPDs were first mandated by the National Fire Protection Association (Pub 58). States were free to adopt that standard or not. Most did. Manufacturers of cylinders immediately adopted it.
But what about filling? If you look at the publication, certified fillers were never
supposed to fill by relying on the OPD valve. Rather, the only two methods ever permitted in the publication were volume or weight: Volume by opening the small screw at the side, and weight by using a repeatedly certified scale. And that makes sense: Fillers were to rely on gravity, which is always there, or on a certified scale, but not on a valve installed who-knows-when and never checked again. (Again, the standards in the publication are voluntary in the sense that individual states are free to adopt them in whole, in part, or not at all.)
So why the OPD at all? Pretty simple. There are lots of folks who fill at home, and those folks were doing a lot of overfilling in the false belief that an overfilled tank was a safe tank. Not!