Oxygen is a modulator of oxygen binding to hemoglobin.

Oxygen is a positive modulator of oxygen binding to hemoglobin.

Explanation:

A positive modulator is one which on binding to an allosteric protein, enhances or increases the activity of that protein.

An allosteric protein is one in which when a ligand binds to one site affects the binding properties of another site on the same protein.

Hemoglobin is a protein that binds and transports oxygen in the body. It exists in two states, the T and R state. It has four binding sites for oxygen. When oxygen is not bound to the heme part of hemoglobin, it exists in the T state with little affinity for oxygen. However, when the first oxygen molecule binds to heme, it brings about conformational changes in the molecule converting it from the T state to the R, and hence it binds with more affinity for the the remaining three molecules of oxygen.

Therefore, oxygen is a positive modulator of oxygen binding to hemoglobin.