IO540 what is the point in using an oxygen bottle "...prior to landing". Unless you are landing at an airfield around 10,000ft amsl then fair enough. Otherwise surely your blood stream would have been replenished with oxygen in the decent below 10,000ft.
I used to fly for about 3hrs at FL120 in an unpressurised aircraft and it was fine really. Been up to FL130 a few times but only for about 20 mins. Wouldn't advise going that high without a gasp of o2