In what form is o2 carried in blood?

In the bloodstream, the majority (over 98%) of oxygen is transported in chemical combination with haemoglobin, a specialised protein found in red blood cells. Oxygen binds reversibly to haemoglobin in the lungs, where the oxygen concentration is high, and is released in tissues where the oxygen concentration is lower. Only a small amount of oxygen (less than 2%) is dissolved in the blood plasma, similar to the way oxygen dissolves in water.