Here's why:

* Oxide is a compound: It's formed by the combination of oxygen with another element. For example, iron oxide (rust) is Fe₂O₃. It doesn't have its own atomic number.

* Atomic numbers refer to elements: The atomic number represents the number of protons in an atom's nucleus, which defines the element.

* Oxygen has an atomic number: The atomic number of oxygen is 8, meaning it has 8 protons in its nucleus.

Therefore, when talking about atomic numbers, you need to specify the element you're referring to.