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):

oppressiveness
    n 1: a feeling of being oppressed [syn: oppression,
         oppressiveness]
    2: unwelcome burdensome difficulty [syn: burdensomeness,
       heaviness, onerousness, oppressiveness]