Question

Is cell using oxygen to break down sugar a physical or chemical change

Answer 1

Breaking down sugar (glucose) is a chemical change. Sugar is a compound that can be broken down.

Answer 2

In your body, "break down sugar" doesn't mean grind up the crystals
into powder, and if it did, you couldn't do it with oxygen, you'd need
a hammer or a grinder.

"Break down sugar" means take a molecule of sucrose or lactose or fructose,
use oxygen to rip it apart, and then put the pieces back together as a molecule
of glucose.

It should be pretty obvious that this is a chemical change.