Here's why:

* Reducing agents donate electrons, causing other substances to gain electrons and be reduced.

* Oxidizing agents accept electrons, causing other substances to lose electrons and be oxidized.

Oxygen is highly electronegative, meaning it strongly attracts electrons. This tendency to gain electrons makes it a powerful oxidizing agent. For example, when oxygen reacts with iron, it pulls electrons from the iron atoms, causing them to oxidize and form iron oxide (rust).

In summary, oxygen is a strong oxidizing agent, not a strong reducing agent.