why is ODC sigmoid in shape?

Oxygen dissociation curve (ODC) or the oxyhaemoglobin dissociation curve plots the relationship of haemoglobin in its saturated form to the prevailing partial pressure of oxygen in the blood. The oxyhaemoglobin dissociation curve is an important tool for understanding how our blood carries and releases oxygen. Oxygen binding to haemoglobin increases with the increasing partial pressure of oxygen but after saturation further binding decreases which results in sigmoidal curve. The main factors which affect the oxygen dissociation curve of haemoglobin are H⁺ concentration,  partial pressure of oxygen and carbon dioxide, temperature, etc. .