Oxygen is an element, meaning it's made up of only one type of atom. It's not made up of smaller elements.

Here's why:

* Elements are the building blocks of matter. They can't be broken down into simpler substances by ordinary chemical means.

* Oxygen is a pure element, represented by the symbol O.

* Oxygen atoms are the smallest unit of oxygen that can exist.

So, oxygen is made of oxygen atoms, and that's it!