Do you tire easily? Do you mean you are short of breath or do you mean you feel very tired. There are many causes for both these symptoms. You need to have a detailed evaluation by your doctor. Please give more details if you need a more focused response.
Likely not oxygen. You are probably describing the sensation of breathlessness which has many other and more likely causes than low oxygen levels. Best to see your physician to evaluate the many more common and benign causes. Deconditioning, anemia, pulmonary and cardiovascular causes may alone or in combination create that sensation.