1. [adv]  happening in a short time or with great speed; "these solutions cannot be found overnight!"
2. [adj]  lasting through or extending over the whole night; "a nightlong vigil"; "an overnight trip"

OVERNIGHT is a 9 letter word that starts with O.

1. \O"ver*night`\, n.
   The fore part of the night last past; the previous evening.
   [R.] --Shak.
   
   

2. \O"ver*night"\, adv.
   In the fore part of the night last past; in the evening
   before; also, during the night; as, the candle will not last
   overnight.
   
         I had been telling her all that happened overnight.
                                                  --Dickens.