If my memory serves me correctly, from my military days:
- Oxygen (air/O2 mix in proportion increasing with altitude) above 10,000ft.
- Unpressurised flight up to 25,000ft. As an aside, British fighter types had a cockpit pressure schedule of "half altitude +2" so a fighter at 40,000ft would have a cockpit altitude of 22,000ft.
- 100% O2 from 25,000ft.
- O2 under pressure above 33,000ft.
- Only Lightning, Starfighter, U2, SR71 and Aurora pilots needed pressure suits (60,000 - 100,000ft).
I stand to be corrected.