How do I help my mom understand that she needs to wear oxygen at night?

Maybe a better approach would be to concentrate on tonight, and not argue about last night. "Of course you are not crazy, Mom. But the doctor says you need to use the oxygen tonight." Or "The doctor said it is not good to go two nights in a row without the oxygen."

It doesn't really matter if she remembers using it in the past. What is important is that she use it tonight.

Is she forgetful about other things as well?

I am not a doctor but I am nurse of 20yrs. I do agree that I wouldn't bring up the previous days as it sounds as though it is only agitating her. I would although just remind her it will help her to sleep and breath more comfortably, and her doctor ordered this for her well being. Sometimes to just applying the oxygen if she has any memory loss? She will accept it without an argument. Good Luck

Does she not want to wear it at night because she doesn't want to or is she forgetting to wear it at night?

If she just doesn't want to and she's of sound mind that's her choice to make. If she's forgetting to put it on prior to bed can you poke your head in the door to say goodnight (and to check that the oxygen is on)? If she's not wearing it then you can remind her.

If she's taking it off in the middle of the night unless you sit by her bedside all night I don't think there's anything you can do.

Always enlist the help of the Dr. And your phrasing wasn't to frightening.

I only have experience with two family members, and they're both strong, determined people. It's been my observation that, like using a cane or walker, ceasing driving, or other health or age related activities/devices, etc., wearing oxygen is to them a clear sign that they're either getting old or losing some capacity as well as better health.

Sometimes strong people resist even more and won't use oxygen until they're actually breathless. This has been my limited experience.

Some years ago a very resourceful woman who did all her own plumbing work, even changed out toilets, also posted on a DIY forum. She wrote that she painted lovely designs on canes and walker legs, and that she sold them at various care facilities. They were a hit.

I decided then and there to decorate my father's walker, perhaps at least with a flag (that's as far as we got though - other priorities prevailed). He did buy a bright red carry-all pouch for his walker and changed it from a drab walker to an item with a bit of panache.

I still would like to festoon his walker with seasonal decorations that are so cheery they draw admiration. Someday...

The DIY woman turned what could visually represent frailty into something of loveliness that older women wanted to use. Dad turned his walker from a metal contraption to one with bright cheery colors that drew compliments from viewers.

I realized then how much the assistive devices were visual demonstration of frailty or aging.

Unfortunately, I haven't figured out to make an oxygen concentrator a thing of beauty, and have not been able to address the issue of which Green Bee writes. I will offer that wearing oxygen at night is an opportunity to get used to it before possibly eventually having to use it 24/7. That's REALLY big change, and not an easy one.

GreenBee, could you perhaps create some activities you can do with your mother before bedtime to help her still feel like a viable and valuable person, someone who still has a lot to offer her family and can still enjoy life, despite wearing oxygen at night?

Perhaps if she goes to bed feeling more confident, more vibrant, and not so much like an older person, then wearing oxygen and remembering if she did or didn't won't be such an issue.

I only tell her she needs her oxygen, and she starts the whole "I don't wear oxygen" talk.
Although Mom is turning 94, she completely cared for herself before an accident a few months ago.
She does have some short term memory, and night is definitely the worst.
My sincere thank you for everyone's ideas. I will try the No, you aren't crazy, etc. Thanks again to all!