FAR Part 135 Federal Aviation Regulations
(3) Whenever a pressurized aircraft is operated at altitudes above
35,000 feet MSL, at least one pilot at the controls shall wear, secured
and sealed, an oxygen mask required by paragraph (b)(2)(i) of this
section.
this is paragraph (b)(2)(i):
(i) At least one pilot at the controls shall wear, secured and
sealed, an oxygen mask that either supplies oxygen at all times or
automatically supplies oxygen
whenever the cabin pressure altitude
exceeds 12,000 feet MSL; and
So the way I read it, you only have to use the O2 mask if you are above
35,000 feet MSL
AND your cabin altitude exceeds 12,000 feet. Am I missing something?