O in WordNet English dictionary

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
   oxygen  O  atomic number 8 
   
2. the blood group whose red cells carry neither the A nor B antigens
   "people with type O blood are universal donors"
   O  type O  group O 
   
3. the 15th letter of the Roman alphabet
   O  o 
   
4. the 15th letter of the Roman alphabet
   O  o 
   

O in English Wiktionary