14 CFR 121.327 says that "cabin pressure altitude" and "flight
altitude" mean the same thing for unpressurized cabins.

Now, even though Part 91 doesn't have the same statement, I think it
reasonable that the same definition applies to 91.211 Supplemental
Oxygen:

---------------snip-------------
At cabin pressure altitudes above 12,500 feet (MSL) up to and
including 14,000 feet (MSL)
---------------snip-------------

What this means is that the oxygen requirements kick in based on MSL
altitudes, not pressure altitudes as is commonly thought, at least for
those of us with unpressurized cabins.


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§ 121.327 Supplemental oxygen: Reciprocating engine powered airplanes.
.....
(d) For the purposes of this subpart "cabin pressure altitude" means
the pressure altitude corresponding with the pressure in the cabin of
the airplane, and "flight altitude" means the altitude above sea level
at which the airplane is operated. For airplanes without pressurized
cabins, "cabin pressure altitude" and "flight altitude" mean the same
thing.