JAR–OPS 1.385 Use of supplemental
oxygen
A commander shall ensure that flight crew
members engaged in performing duties essential to
the safe operation of an aeroplane in flight use
supplemental oxygen continuously whenever cabin
altitude exceeds 10 000 ft for a period in excess of
30 minutes and whenever the cabin altitude exceeds
13 000 ft.
ANO article 54 (7) (c) during any period when the aircraft is flying above flight level 100 oxygen is used
by all the flight crew of the aircraft.
The above applies to public transport flights. I don't know of any mandatory requirements for private flights.