o (Noun) — The blood group whose red cells carry neither the A nor B antigens. ex. "people with type O blood are universal donors"
o (Noun) — The 15th letter of the Roman alphabet.
o (Noun) — A nonmetallic bivalent element that is normally a colourless, odourless, tasteless, nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust.
o (Interjection) — Used to express surprise. ex. "O, it was only a dollar last time I was here!"