Synonyms for O (same or very similar meaning)
WordNet sense 1 (a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust):
oxygenWordNet sense 2 (the 15th letter of the Roman alphabet):
letter oWordNet sense 3 (the blood group whose red cells carry neither the A nor B antigens):
group O, type OFrom the ODE community, based on WordNetadd/edit