At TOD into LGW with an empty B777 with just myself and my co-pilot on board we had to don oxygen masks and remained wearing them until after landing.
A couple of issues:
The deafening noise when breathing if the intercom is NOT switched off is a major distraction. Continually trying to co-ordinate breathing/talking and switching is an unwelcome distraction in a demanding situation.
Wearing the full face EROS mask is not something a pilot experiences very often and the isolation it engenders is quite a surprise. e.g. it is almost impossible to look face to face at your colleague, your communication is entirely conducted via the intercom. Not an impossible situation but one rarely experienced in the civilian world.