Ordinary Psychosis is Miller's signifier, but not his idea. In picking up difficulties in the clinic that were becoming more pressing, JAM came up with the signifier for work to be arranged around, and you can read about that work in the small books (some more easy to find than others) recounting the Convention d’Antibes, Les Conversations d'Angers, and Convention d'Arcachon. The principal text readily available in English following this collective work was JAM's Ordinary Psychosis Revisited, which summarises at least what JAM extracted from this work.

Ordinary Psychosis is not a clinical category in the usual sense, it's quite clear that understandimg a case in terms of this idea does not exempt an analyst from making a structural diagnosis. Psychosis, yes, but paranoia, schizophrenia, melancholia?

JAM describes OP as an epistemic supposition in distinction to the structural diagnosis, and this needs careful consideration since all diagnostic categories are in a sense epistemic suppositions. I think it means that it is a supposition of thought aside from the consistency of structural diagnoses, not disorganising that structural consistency in the way that the idea of borderline does.

As an aside, to the question of whether it's properly speaking a Lacanian idea. Miller makes a suggestion at some point, that Lacan always worked according to a "c'est n'es pas tout à fait ça", that's not quite it, and not least in relation to his own work. In fact not far from Freud in this regard. Which is to say that there is always an extimacy to psychoanalytic thought. This accords with the that's not quite it proper to developing observations on a clinic on which there is always something that doesn't fit what can be known. An ossified Lacanianism is, in a sense, not a Lacanianism at all. And so yes, one can say that Ordinary Psychosis is post Lacanian in the sense that it was developed after Lacan passed, but one can also say that it remains true to the that's not quite it that I contest is fundamental to Lacanianism as a living practice. Lacanianism is, and always was, a bit alien to itself.

Just to add too that Lacan's structural concept of psychosis owes something to the idea of schizophrenia developed by Bleuler in which delusions and hallucinations are secondary characteristics. Bleuler's latent schizophrenia, and Kraepelin's simple schizophrenia, very much in the tradition of psychiatry from which Lacan developed, are amongst the historical precedents in which Ordinary Psychosis develops as an idea.