For what it's worth.
FAA regs permit operation up to 12,500 ft without supplemental oxygen, and up to 14,000 ft for 30 minutes.
I instructed at a flight school where we exposed students to aircraft operation at 12,000 ft without oxygen. All students remarked on a perceived change in capability. Approximately 50% made significant navigational errors and all of those were sure that they hadn't made errors. The errors started occurring after about half an hour.
So, I guess even if you are operating the aircraft with oxygen, you may get your planning on the ground wrong and not notice.