The oxygen requirements only come into play when you fly depressurized.
That is true for the 12,500, 14,000, and 15,000 feet cabin pressure numbers I gave, yes.
However, the oxygen requirement associated with the 35,000 foot number I gave applies to both pressurized and unpressurized flight. If your aircraft is at or above 35,000 feet pressure altitude, at least one of the pilots must be on oxygen, or must have a quick-donning mask available at all times.
About the only requirement is that it is legible. Your "distinctive" handwriting could become part of your brand.