Oxyhaemoglobin is formed when haemoglobin combines with oxygen. Oxyhaemoglobin is basically present in the oxygenated blood and is formed during physiological respiration when oxygen binds to the haem component of the protein haemoglobin in RBCs.
Haemoglobin has four haem molecules held by intermolecular bonds and the oxygen molecules bind to the iron(II) ions of the haem to form oxyhaemoglobin.
Hb + 4O2 --> Hb(O2)4
One hemoglobin carries four oxygen molecules.