Yes, I remember that drained feeling. When my mom started to have serious health issues, my father and stepfather were long gone. (She married older men.) But if the two parents are similar in age, it can be quite hard on one when the other one falters. The sight of your mom on oxygen must have surprised and upset him. To compound the problem, sometimes doctors can be very downbeat and paint the most negative picture of the situation. You and your siblings will have to help him navigate through this. Having a strong and capable partner beside him is very important to him, but he did promise to live "in sickness and in health" with her, so he will need to find the courage to go forward. Old age ain't for sissies!!
And you will need to find a way to get through this without being overly stressed. For me, I stopped on the way to seeing her and had a therapeutic bulgogi bowl, working a crossword puzzle, listening to calming music, and puttering in my garden when I got home. I curled up with my cat and read a good book. I feasted my eyes on glorious sunrises and sunsets and drove to the mountains. I don't recommend staycations at this point; you need to do more of a complete reset occasionally. But sometimes a staycation works if you work in special events such as a movie you've always wanted to see, a visit to a museum, or visits with friends.