Big John wrote
As I recall there are some times when one of the pilots has to wear a
mask. Is it above a certain altitude or some other condition (like one
pilot leaving the cockpit???)

Yes...above FL240, if one pilot leaves the cockpit, the remaining
one must wear the mask and use the oxygen. From my own personal
experience, it was not very well enforced at any airline. At PanAm,
the PIC was encouraged to drill the crew in donning the masks with
one hand in less than the required 5 seconds.

Bob Moore