Of course the name Ossian/Oisin is familiar from the Finn MacCool tales.
My focus is something fairly particular in British history.

In the time of England's Dr. Samuel Johnson, a man named James MacPherson published English verses for which he made some provocative claims. MacPherson claimed, that these verses were not simply his own poems about Oisin and the Fianna Fail, but that they had origins in the Scottish Gaelic and that he was the translator. I mention Dr. Johnson because this assertion roused the latter's temper. As I grasp it, Dr. Johnson proceeded, in the colloquial phrase, to toss the baby out with the bathwater, and insisted that the whole darned thing was false. Maybe I have that wrong, but quotes from Johnson are certainly very negative.

Well, that was a century or more ago,
and in the meanwhile there has been a lot of hard work
at locating manuscripts preserved, for example, in monasteries by monks,
and there is much more known today about source materials and languages,
and about their contents,
than was general knowledge in the time of Dr. Johnson and MacPherson.

So why bring up the controversy peculiar to them?
Because:
that literary event, however dubious or made-up,
had tremendous consequences, not in the "trad" world,
but in continental European literature and art.

My specialty and love is classical vocal music;
and translations into modern European languages,
of MacPherson's English,
became all the rage in Europe.
It is asserted that German Romanticism would have been different
without the Ossianic myth to excite poets and composers alike,
and indeed you can find,
not just Mendelssohn ("Fingal's Cave")
but Schubert and Brahms attracted to the whole Ossianic idea.

This subject may be distasteful, which I regret,
but as I say,
it is a matter of literary and musical history.
And the more I can sort out
what is authentic,
what is stolen,
and what is invented and presented as something other than it is,
the more it would please me. Thanks.