While events may not be exclusive to ORAS in the sense that there is content involving the event Pokemon and they can be traded to other games in said gen, There have been some events, in the past (American releases of Genesect for example), that exclude earlier versions of the generation in terms of distribution. As in, Yeah we're giving out Volcanion, but you can only download it on ORAS so sucks for you people who only bought XY. Typically events seem to be made available to all games in the current gen though.With the exception of Meloetta (who got paired up with Keldeo's movie in a short), event legendaries are staggered by a year at a time.
By virtue of this they're almost always going to be revealed prior to a game, but the scheduling means we can get cases like now where we still need 2 of them before ORAS
eg: Manaphy had to be prior to DP, Darkrai before Platinum, Shaymin JUST before Platinum, Arceus before HGSS
So while I wouldn't be surprised if they had their own events (eg: if you took your Arceus to various places in Sinnoh you got backstory, if you took it to the unown ruins you got that special event), I doubt they'll be exclusive to any given game.
Not that it really matters, it's not like they do old-fashioned events anymore.
R_N is right about the historical pattern of distributing new events, but we get repeat events in between at all kinds of random times. So your hopes may be fulfilled. Deoxys is actually one of the more recent guys to get a repeat event (although Japan is getting a shiny Jirachi event right now) but I think we can expect a US Jirachi event sometimes soon too, as it's been quite awhile since we got one here, Japan just got one, and Jirachi's gen is getting remade.If anything, I'd be all for a Jirachi and Deoxys event leading up to ORAS as I have technically "completed" the pokedex in Y barring Mew, Deoxys, Jirachi, Shaymin, Darkrai, Arceus, Victini, and Meloetta. Isn't there an upcoming Diancie event as well?
Diancie's event begins in less than 3 days (especially factoring in time zones) it starts on the 19th but I was under the impression that for now it is a Japan only thing that coincides with buying a movie ticket to see Diancie's motion picture.