Bit late to the discussion here, but OOTB was doing the rounds as a white label, then as a promo for a while before the 'official' release.That's an interesting find.
Out Of The Blue was released 02.03.1999 in Netherlands but recorded somewhere in 1998.
My money's on Ferry Corsten to be the first one (as he was a genius full of creativity in the 90's), although I still want to make a proper research about this.
Not sure on the timeline, but I imagine if the full release was early 1999, then the white labels then promos would have been around 1998.
A&R was very aggressive on jumping on popular tunes back then, so I imagine a lot of these 'clones' may well have been at request of the labels to cash in on the OOTB hype by getting their similar tracks released before-hand.
I remember there being some controversy when Anjunabeats released the Rollerball - Albinoni track, which was based on Albinoni's adagio.
There was another track based off the same piece that beat the Rollerball track to the release and I remember Tony McGuinness getting quite annoyed about it. I cant remember the name of the tune, 'Epica' or something similar rings a bell - maybe someone can remember.