Explain why potassium iodide is used to test for oxidising agents.

Identify the oxidizing agent and reducing agent in the given reaction, by looking at the gain and loss of oxygen : 

$2{\mathrm{Mg}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)} \to 2{\mathrm{MgO}}_{\left(\mathrm{s}\right)}$