The Collaborative International Dictionary of English v.0.48:

Oppress \Op*press"\, v. t. [imp. & p. p. Oppressed; p. pr. &
   vb. n. Oppressing.] [F. oppresser, LL. oppressare, fr. L.
   oppressus, p. p. of opprimere; ob (see Ob-) + premere to
   press. See Press.]
   1. To impose excessive burdens upon; to overload; hence, to
      treat with unjust rigor or with cruelty. --Wyclif.
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            For thee, oppress[`e]d king, am I cast down. --Shak.
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            Behold the kings of the earth; how they oppress
            Thy chosen!                           --Milton.
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   2. To ravish; to violate. [Obs.] --Chaucer.
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   3. To put down; to crush out; to suppress. [Obs.]
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            The mutiny he there hastes to oppress. --Shak.
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   4. To produce a sensation of weight in (some part of the
      body); as, my lungs are oppressed by the damp air; excess
      of food oppresses the stomach.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oppress
    v 1: come down on or keep down by unjust use of one's authority;
         "The government oppresses political activists" [syn:
         oppress, suppress, crush]
    2: cause to suffer; "Jews were persecuted in the former Soviet
       Union" [syn: persecute, oppress]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "oppress":
   abuse, addict, afflict, aggrieve, anguish, bear hard upon,
   beat down, beset, break, break down, bring to tears, browbeat,
   bulldoze, bully, burden, burthen, busy, cast down, castrate,
   charge, clamp down on, coerce, compel, conquer, cow, crush,
   crush one, cumber, cut up, damp, dampen, dampen the spirits,
   darken, dash, daunt, deject, depress, desolate, despotize,
   discourage, dishearten, dispirit, distress, domineer,
   domineer over, draw tears, drive, embitter, encumber, enslave,
   exhaust, fag, freight, go hard with, go ill with, grieve, grind,
   grind down, hamper, handicap, harass, harry, haunt,
   haunt the memory, henpeck, hound, hunt, intimidate, inundate,
   keep busy, keep down, keep under, knock down, lade, lie on, load,
   lord it over, lower, lower the spirits, maltreat, molest, obsess,
   outrage, overawe, overbear, overburden, overcome, overdrive,
   overload, overmaster, overpower, override, overtask, overtax,
   overthrow, overweight, overwhelm, overwork, persecute, press,
   press down, press hard upon, press heavy on, pressure, prey on,
   prostrate, pursue, put down, repress, rest hard upon, ride over,
   ride roughshod over, sadden, saddle, sink, sorrow, subdue,
   subjugate, suppress, sweat, task, tax, terrorize, tire, torment,
   torture, trample down, trample underfoot, trample upon, tread down,
   tread upon, trouble, try one, tyrannize, tyrannize over, unman,
   victimize, walk all over, walk over, wear down, wear out,
   wear upon one, weary, weigh down, weigh heavy on, weigh heavy upon,
   weigh on, weigh upon, weight down, work