A 2017 study had type-1 diabetics breathe extra oxygen.
Blood pressure and arterial stiffness increased, indicating a rise in oxidative stress.
Then, they had them breathe extra oxygen again, but this time while breathing slowly at 6 breaths a minute.
Slow breathing offset the harmful effects of extra oxygen on blood pressure and arterial stiffness.
Leading to this hypothesis: “…our results lead to the hypothesis that slow breathing may exert some antioxidant effect, possibly via parasympathetic stimulation.”
Pretty neat.