The Collaborative International Dictionary of English v.0.48:

Onus \O"nus\, n. [L.]
   A burden; an obligation.
   [1913 Webster]

   Onus probandi[L.], the obligation to furnish evidence to
      prove an assertion; the burden of proof.
      [1913 Webster]

WordNet (r) 3.0 (2006):

onus
    n 1: an onerous or difficult concern; "the burden of
         responsibility"; "that's a load off my mind" [syn:
         burden, load, encumbrance, incumbrance, onus]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "onus":
   absolute indication, allegiance, arraignability, arraignableness,
   aspersion, assigned task, attaint, badge of infamy, bar sinister,
   baton, bend sinister, black eye, black mark, blameworthiness,
   bloody hands, blot, blur, bounden duty, brand, broad arrow, burden,
   burden of proof, burthen, business, call of duty, cargo,
   censurability, censurableness, censure, champain, charge,
   chargeability, commitment, complicity, conclusive evidence,
   criminality, cross, culpability, cumbrance, damning evidence,
   deadweight, dedication, deference, demonstration, determination,
   devoir, devotion, difficulty, dirty hands, disadvantage,
   disparagement, duties and responsibilities, duty, embarrassment,
   encumbrance, establishment, ethics, fault, fealty, freight, guilt,
   guiltiness, guilty conscience, hamper, handicap, homage,
   impeachability, impeachableness, impediment, impedimenta,
   imperative, implication, imposition, imputation,
   incontrovertible evidence, inconvenience, inculpation,
   indictability, indictableness, indisputable evidence, involvement,
   ironclad proof, line of duty, load, loyalty, lumber, mark of Cain,
   millstone, mission, must, obligation, odium, onus probandi, ought,
   pack, peccancy, penalty, pillorying, place, point champain, proof,
   red-handedness, reflection, reprehensibility, reprimand, reproach,
   reproachableness, reprovability, reprovableness, respect,
   self-imposed duty, settlement, slur, smear, smirch, smudge, smutch,
   spot, stain, stigma, stigmatism, stigmatization, sure sign, taint,
   tarnish, task, tax, trouble, unmistakable sign, weight,
   white elephant