It is hemoglobin that is the oxygen transport substance in blood. Wikipedia has some good information on this metalloprotein, and a link is provided.
Oxygen enters the body by being absorbed into the blood (specifically, it is absorbed by red blood cells which contain an oxygen-carrying compound known as hemoglobin) when air is inhaled into the lungs, which have a highly porous, spongy structure which facilitates the exposure of blood to the air.
Haemoglobin
a colorless protein obtained by removing heme from hemoglobin; the oxygen carrying compound in red blood cells
Hemoglobin
Hemoglobin is the oxygen carrying molecule in red blood cells.
White blood cell
Red blood cells
Red blood cells, specifically the protein hemoglobin within red blood cells, is responsible for carrying oxygen in the blood. Hemoglobin binds to oxygen in the lungs and transports it to tissues throughout the body.
then they are red if they are not carrying oxygen then they are a purple red.
Blood cells that are carrying oxygen will appear bright red, while blood cells that are not carrying oxygen will appear darker in color (usually dark red or purplish). This change in color is due to the presence or absence of oxygen-binding molecules like hemoglobin within the red blood cells.
hemoglobin
Erethrocytes