Is oxygen carried through the blood stream dissolved in plasma? Is it carried on receptors on the red blood cell membrane?
No, and no to your 2 questions
(Strictly speaking, there will be small amounts of oxygen carried dissolved in the plasma, but for exam purposes the following is correct)
The principal way in which oxygen is carried around the body is bound to haemoglobin molecules (as oxyhaemoglobin) inside the red blood cells. These molecules disassociate freely in high CO2 concentrations, so oxygen is released from the red blood cells especially in actively respiring tissues