The ANO Schedule 4 Scale L1 Part 1, refers to the requiremnts for the carriage of oxygen. In short, in a Transport aircraft, the pilots must be on oxygen above pressure altitude of 10,000 feet (cabin altitude). However, the pax can be at between 10,000 feet and FL130 for 30mins without oxygen. Assuming the masks all worked in the back during the decent, I see no reason why you could not level at FL130 for 30 mins (max, and including decent to FL100), so long as the pilots have sufficient for that time.