A protein called haemoglobin in the red blood cells. The haemoglobin can loosely bond with oxygen allowing it to transport oxygen from the lungs to the other parts of the body.
There is a condition called anemia which causes a reduction or malfunction of hemoglobin which is the oxygen carrying molecule within a red blood cell. There can be several causes of anemia such as...
Yes - oxygen is held in red blood cells (in haemoglobin to be precise). As the blood flows, oxygen is brought all around the body and eventually gets back to the heart and lungs as carbon dioxide...
Hemoglobin is a metalloprotein, containing iron, that is within red blood cells and binds to gasses like oxygen in order to transport those gasses in the blood.