Hemoglobin
Oxygen enters the blood stream via the erythrocyte (Red blood cell, RBC), I am not sure what you are asking but the hemoglobin inside the RBC has 4 "oxygen holder molecules" called Heme's where an iron molecule (Fe++) is stored, the Fe++ is what oxygen attaches to.
Blood is made inside your bones, and "refills" on oxygen inside your lungs.
Haemoglobine combines with oxygen to form oxihaemoglobine
Carbon monoxide combines with hemoglobin in the blood to form carboxyhemoglobin, which reduces the blood's ability to carry oxygen.
Hemoglobin is the compound in red blood cells that carries oxygen from the lungs to body cells. The oxygen combines readily with the ion in hemoglobin, and hemoglobin can carry more than twenty times its own volume in oxygen. After releasing oxygen to the cells, hemoglobin collects carbon dioxide and carries it to the lungs where it is exhaled.
Blood is ~55% plasma and ~45% red blood cells (erythrocytes). These cells contain millions of molecules of the protein hemoglobin each of which can bind up to 4 oxygen molecules.
Hemoglobin
Hemoglobin combines readily with oxygen.
The compound that binds to oxygen in red blood cells is hemoglobin. Hemoglobin is a protein found in red blood cells that has a high affinity for oxygen, allowing it to transport oxygen from the lungs to the tissues in the body.
Hemoglobin is the iron containing protein in our red blood cells that combines with and carries oxygen.
haemoglobin a comles compound of protein with iron
When oxygen combines with red blood cells, it binds to the hemoglobin protein within the cells to form oxyhemoglobin. This process occurs in the lungs, where oxygen is loaded onto hemoglobin. The oxyhemoglobin then travels through the bloodstream to deliver oxygen to tissues throughout the body.