Opposite words for hello:
bon voyage, congé, adieu, farewell, good-bye, godspeed.
Usage samples:
  1. " Hello, Milty," she said.
    "Out Like a Light", Gordon Randall Garrett.
  2. " Hello, Betsy, you're just in time.
    "Understood Betsy", Dorothy Canfield.
  3. No. Instead: " Hello, sweetheart!"
    "Half Portions", Edna Ferber.
  4. When directly under the window, the " Kid" stuck his head out, saying: " Hello, Bob!"
    "History of 'Billy the Kid'", Chas. A. Siringo.