I have a friend who uses oxtgen and has developed a sore under her nose. is there anything she can put on it without interferring with the oxygen?

Yes: It could be a yeast or bacterial infection. If you think it's just from the tubing rubbing the skin, you could cut a small strip of moleskin (in the foot section if your pharmacy) & stick it to the tubing to protect the skin. It should be changed every day or two. A good moisturizer will also protect the skin. Check w the respiratory tx company for other ideas. Or w her doc, esp if infected.