An interesting resource that I found on the topic of breathing is https://www.normalbreathing.com/
Dr Artour Rakhimov claims that breath starvation (breathing slowly and less) is key to higher oxygen absorption; and definitely not deep breathing! "Take a deep breath" makes sense if you have been hyper-ventilating, but not ALL the time.
I found his site via his "How to unblock a blocked nose in 1 minute" video, a technique which works even when both nostrils are clogged: