The Collaborative International Dictionary of English v.0.48:

Oppressive \Op*press"ive\, a. [Cf. F. oppressif.]
   1. Unreasonably burdensome; unjustly severe, rigorous, or
      harsh; as, oppressive taxes; oppressive exactions of
      service; an oppressive game law. --Macaulay.
      [1913 Webster]

   2. Using oppression; tyrannical; as, oppressive authority or
      commands.
      [1913 Webster]

   3. Heavy; overpowering; hard to be borne; creating a sense of
      heavy burden; as, oppressive grief or woe; oppressive heat
      or humidity; an oppressive workload.
      [1913 Webster +PJC]

            To ease the soul of one oppressive weight. --Pope.
      [1913 Webster] -- Op*press"ive*ly, adv. --
      Op*press"ive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oppressive
    adj 1: weighing heavily on the senses or spirit; "the atmosphere
           was oppressive"; "oppressive sorrows"
    2: marked by unjust severity or arbitrary behavior; "the
       oppressive government"; "oppressive laws"; "a tyrannical
       parent"; "tyrannous disregard of human rights" [syn:
       oppressive, tyrannical, tyrannous]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "oppressive":
   Herculean, absolute, absolutist, absolutistic, agonizing, airless,
   annoying, arbitrary, arduous, aristocratic, arrogant,
   authoritarian, authoritative, autocratic, backbreaking, besetting,
   black, bleak, bossy, bothersome, breathless, breezeless, brutal,
   burdensome, close, crushing, cumbersome, cumbrous, demanding,
   depressant, depressing, depressive, despotic, dictatorial,
   discouraging, disheartening, dismal, dispiriting, distressing,
   dolorous, domineering, down, effortful, encumbering, exacting,
   exhausting, exigent, feudal, forced, grievous, grinding, grueling,
   hampering, hard-earned, hard-fought, harrowing, harsh, heavy,
   hefty, high-handed, humid, ill-ventilated, impedimental, impeding,
   impeditive, imperative, imperial, imperious, incumbent,
   intolerable, irksome, killing, labored, laborious, lordly, lumpish,
   magisterial, magistral, massive, masterful, miserable, monocratic,
   muggy, onerous, operose, overbearing, overburdensome, overpowering,
   overruling, overwhelming, painful, peremptory, plaguey, punishing,
   racking, repressive, severe, somber, sticky, stifling, strained,
   strenuous, strict, stuffy, suffocating, sultry, superincumbent,
   suppressive, taxing, toilsome, tough, troublesome, trying,
   tyrannical, tyrannous, unbearable, uncomfortable, unendurable,
   unvented, unventilated, unwieldy, uphill, vexatious, wearing,
   wearisome, wearying, weighty, windless, wretched