If it was just 5 seconds, we wouldn’t notice changes in our breathing.
But do you know what would happen around us?
Earth’s crust contains 45% oxygen. Without oxygen, crust would crumble, causing the ground to crumble and we would be in freefall. Buildings, bridges and concrete structure would crumble into dust, as oxygen is the binding agent for concrete. Cars would stop, and planes would fall from sky as their combustion engines wouldn’t work without oxygen. Also, losing oxygen means losing almost 21% air pressure. This would cause our inner ear to explode, causing hearing loss. It would also become darker suddenly.
Do you know why?
For sunlight to reach us, it needs to bounce off air particles like oxygen, dust etc. No oxygen means much fewer particles to bounce off, thus much darker.