The element that reacts with a metal or non-metal to form an oxide is oxygen.

Here's why:

* Oxidation: The process of forming an oxide involves the addition of oxygen atoms to another element.

* Metals: Metals tend to lose electrons and form positive ions (cations). When they react with oxygen, they form metal oxides. For example, iron reacts with oxygen to form iron oxide (rust).

* Non-metals: Non-metals tend to gain electrons and form negative ions (anions). When they react with oxygen, they form non-metal oxides. For example, sulfur reacts with oxygen to form sulfur dioxide.

So, in general, oxygen is the key element involved in the formation of oxides.