Problem with the OHJ is that it's quite possible for more than one aircraft to arrive in the overhead at the same altitude, at the same time, from different directions. Many pilots think all they need to do is to make the standard "book" calls and that is all. They believe they must be safe, because they are following the "book" procedure.
They might not be safe..... all the OHJ does is move the point of possible aircraft / aircraft conflict. Ever been in the situation where you've called "Overhead" and begun the descent only to hear another aircraft also call "Overhead" immediately afterwards? From personal experience, it quite rightly makes your hair stand on end. Is the second joining aircraft above, or below? Is he/she on the correct altimeter setting yet? Are they aware of your presence?
I've had this happen more than once in the four decades I've been flying (and instructing). Once I was supervising a PPL from the right seat when this happened. Unknown to us, another aircraft was following us very close in. The pilot had been doing so for some miles and was gaining, but made no effort to keep his distance, nor a previous call to advise other pilots of the fact that he was joining at that airfield (we had). The "overhead" call was the first call he made.
Obviously, there could also be a non-radio "joiner" up there at any time.
Maybe the OHJ protagonists would put forwards their opinion of what the "book" action is, in this instance....
The real answer is to look out, listen out, and use the radio to communicate
properly, and to use your noggin, not merely "follow the book".
(And don't formate without previous agreement, in particular in the overhead!).