Source : Webster's Revised Unabridged Dictionary (1913)

Oppressor \Op*press"or\, n. [L.]
   One who oppresses; one who imposes unjust burdens on others;
   one who harasses others with unjust laws or unreasonable
   severity.

         The orphan pines while the oppressor feeds. --Shak.

         To relieve the oppressed and to punish the oppressor.
                                                  --Swift.

Source : WordNet®

oppressor
     n : a person of authority who subjects others to undue pressures