I'll make an attempt at this; probably some of the others know more than I do about it though.
Oil-hardening doesn't have to have quite as fast a quench to harden as the water-hardening does. (Water cools better/faster than oil.) Therefore, you can probably quench oil-hardening in water, although it may warp or crack more than it would if quenched in oil, but if you try to quench water-hardening in oil it may not harden as much as it should.
At least that's my guess....
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Everybody is ignorant, only on different subjects. -- Will Rogers
Law of Logical Argument - Anything is possible if you don't know what you are talking about.