The oldest movements that are fully stocked in terms of parts would be the 55X/56X series, and the 600 series equivalent in a manual wind. So currently pretty much all parts for these are available from Omega, but it's unclear how much longer this will be the case.
For anything older than that, generally the further in time you go back, the less likely common wear parts will be available - since the common parts that wear will be the first to have remaining stocks used up once they stop producing the parts.
So the answer to your specific question is no, the 284 doesn't have all parts available for it - far from it. For example of the wheel train the only one available from Omega is the fourth wheel, so no great wheel, third wheel, escape wheel, pallet fork, etc. You or your watchmaker may have to search for what's needed on the secondary market, which takes time.
Cheers, Al
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