The Air Transporation Safety Board of Canada has prepared one of the best documented cases of a hypoxia induced airplane crash.
The case documented is of a 2018 PA-31 twin engine Piper that flew between 13k and 15k. The pilot appears to have inadvertently run out of oxygen during flight, which led to a slow degradation of mental faculties and strange flight inputs. Most of the flight was captured on a flight recorder, so the documentation is significant and it is alarming.
Essentially, the pilot lost the ability to control the aircraft, lost power to one engine, entered a spin through IMC, was unable to maintain headings or airspeed, and only reported an emergency right before impact.
The report notes that hypoxia is an insidious and deadly killer that isn't well trained or regulated for. Pilots in Canada and the U.S. are essentially on their own as hypoxia training isn't required for unpressurized aircraft.
You can see additional charts, visuals of the flight path, and report information here: http://www.tsb.gc.ca/eng/rapports-reports/aviation/2018/a18w0116/a18w0116.html