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.
      [1913 Webster]

            For thee, oppress[`e]d king, am I cast down. --Shak.
      [1913 Webster]

            Behold the kings of the earth; how they oppress
            Thy chosen!                           --Milton.
      [1913 Webster]

   2. To ravish; to violate. [Obs.] --Chaucer.
      [1913 Webster]

   3. To put down; to crush out; to suppress. [Obs.]
      [1913 Webster]

            The mutiny he there hastes to oppress. --Shak.
      [1913 Webster]

   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]

The Collaborative International Dictionary of English v.0.48:

oppressed \oppressed\ adj.
   having excessive or unfair burdens imposed.

   Syn: downtrodden, persecuted.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

oppressed
    adj 1: burdened psychologically or mentally; "laden with grief";
           "oppressed by a sense of failure" [syn: laden,
           oppressed]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "oppressed":
   abused, bond, browbeaten, burdened, captive, charged, cumbered,
   disenfranchised, downtrod, downtrodden, encumbered, enslaved,
   enthralled, fraught, freighted, ground down, hampered, heavy,
   heavyhearted, henpecked, in bondage, in bonds, in captivity,
   in chains, in leading strings, in slavery, in subjection, laden,
   laden with sorrow, loaded, long-faced, misused, ordered around,
   overborne, overburdened, overcharged, overfraught, overfreighted,
   overladen, overloaded, overtaxed, overweighted, regimented, sad,
   sad of heart, sad-eyed, sad-faced, saddened, saddled, sadhearted,
   servile, slavish, subjected, subjugated, submissive, suppressed,
   taxed, trampled, tyrannized, under the heel, under the lash,
   unfree, unmanned, weighed upon, weighted, weighted down