You need more oxygen, not less, to think and act appropriately, so train yourself to inhale energy and exhale tension. Shallow breathing is a typical physical response to anger, stress, fear, sadness, and pain. It takes a conscious effort to breathe deeply when you're flooded by emotion, but by doing so you can control the spiral of emotion and physical response. Calm, steady breathing is a lullaby for body and brain - a signal that everything really is okay. When practicing deep breathing, inhale through your nostrils, because they filter and warm the air before it enters your delicate lungs.
Some people prefer to exhale through their nostrils, while others exhale through the mouth.
This is a personal preference, and it doesn't matter. Focus on the sound of your breath and what you feel as you breathe. Think about inhaling energy and exhaling tension. Breathing is the most important thing you do in this minute, in the next hour, all day today, and any day in the future. How privileged you are to be able to breathe.