The Collaborative International Dictionary of English v.0.48:

Oppression \Op*pres"sion\, n. [F., fr. L. oppressio.]
   1. The act of oppressing, or state of being oppressed.
      [1913 Webster]

   2. That which oppresses; a hardship or injustice; cruelty;
      severity; tyranny. "The multitude of oppressions." --Job
      xxxv. 9.
      [1913 Webster]

   3. A sense of heaviness or obstruction in the body or mind;
      depression; dullness; lassitude; as, an oppression of
      spirits; an oppression of the lungs.
      [1913 Webster]

            There gentle Sleep
            First found me, and with soft oppression seized
            My drowsed sense.                     --Milton.
      [1913 Webster]

   4. Ravishment; rape. [Obs.] --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oppression
    n 1: the act of subjugating by cruelty; "the tyrant's oppression
         of the people" [syn: oppression, subjugation]
    2: the state of being kept down by unjust use of force or
       authority: "after years of oppression they finally revolted"
    3: a feeling of being oppressed [syn: oppression,
       oppressiveness]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "oppression":
   McCarthyism, Schmerz, Slough of Despond, abuse, affliction,
   airlessness, anguish, argumentum baculinum, bale, big stick,
   bitter cup, bitter draft, bitter draught, bitter pill,
   blank despondency, burden, burden of care, burdening, burthen,
   cankerworm of care, care, cargo, charge, charging, closeness,
   cross, crown of thorns, cumber, cumbrance, curse, deadweight,
   death wish, dejectedness, dejection, depression, despair,
   despondency, despondentness, despotism, discouragement,
   disheartenment, dispiritedness, distress, domination, domineering,
   down trip, downcastness, downer, downheartedness, drag,
   drooping spirits, encumbrance, enslavement, fascism, freight, gall,
   gall and wormwood, grievance, handicap, harassment, hardship,
   harrying, heartlessness, heavy hand, high hand, hopelessness,
   hounding, incubus, incumbency, infliction, injury, injustice,
   iron boot, iron hand, iron heel, lading, load, loading,
   low spirits, lowness, lowness of spirit, malaise, maltreatment,
   millstone, oppressiveness, overload, overtaxing, overweighting,
   pack of troubles, pain, peck of troubles, persecution, pessimism,
   pressure, red-baiting, reign of terror, repression, saddling,
   sea of troubles, self-destructive urge, sinking heart, sorrow,
   spiritlessness, stuffiness, subjection, subjugation,
   suicidal despair, superincumbency, suppression, surcharge,
   taedium vitae, taxing, terrorism, thorn, thought control, torment,
   tormenting, torture, trouble, tyranny, victimization,
   waters of bitterness, weariness of life, weight, witch-hunt,
   witch-hunting, woe