O
WordNet
noun
(1) The blood group whose red cells carry neither the A nor B antigens
"People with type O blood are universal donors"
(2) The 15th letter of the Roman alphabet
(3) 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
WiktionaryText
Letter
- The fifteenth letter of the basic modern Latin alphabet.
Symbol
O
Letter
- A blood type that lacks A or B antigens and may only receive transfusions of similar type O blood, but may donate to all (neglecting Rh factor). Synonym: universal donor.
- American Library Association
Particle
-
- O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you? -- Galatians 3:1
Usage notes
This is one of the few grammatical particles English has, and it is borrowed via the various translations of the Bible. Irish is the last Indo-European language which does have it (as a full-blown grammatical case, unlike English, not resurrected but rather preserved), and indeed does it with the capital O.