Hi abbey, and thanks for your comment! You've posed an excellent question which I hope to be able to address.
First, there are ways to insure that one's cannula, for use with BiPAP at night, can be made to fit more securely. If this could be accomplished on your behalf, that will provide one part of a solution. The medical equipment company, which provides the BiPAP machine, must have clinicians on staff who can assist with this. Hopefully the clinical respiratory therapist will be able to help you to insure a proper fitment for your cannula device. That is the most important aspect of using BiPAP - the proper fitment of the patient's device!
Secondly, if you are using a pulse oximeter in your home, that is the proper way to monitor one's oxygen saturation level (at home), as you well know. There are many of these devices commercially available in the market place, nowadays. The good news is that some of them have integrated alarms. If you do your own GOOGLE search using the words 'pulse oximeter with alarm', you will see a wide selection and price range of these devices.
I hope this brief reply guides you in the right direction to resolve your real-world concerns. Please do check back and let us know how this all turns out for you.
You are always welcome here in our online community.
Wishing you well,
Leon (site moderator COPD.net)