What to do if i have o positive blood which is what they want, why can't i donate my blood?

More criteria: Blood donors need to meet certain criteria to meet eligibility requirements for donation, e.g., hemoglobin level, lack of high risk activities, absence of transmissible diseases etc. See this site for more info and you should ask the blood bank that declined you as a donor. http://www.redcrossblood.org/donating-blood/eligibility-requirements.