The Collaborative International Dictionary of English v.0.48:

Oblige \O*blige"\ ([-o]*bl[imac]j"; 277), v. t. [imp. & p. p.
   Obliged ([-o]*bl[imac]jd"); p. pr. & vb. n. Obliging
   ([-o]*bl[imac]"j[i^]ng).] [OF. obligier, F. obliger, L.
   obligare; ob (see Ob-) + ligare to bind. See Ligament,
   and cf. Obligate.]
   1. To attach, as by a bond. [Obs.]
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            He had obliged all the senators and magistrates
            firmly to himself.                    --Bacon.
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   2. To constrain by physical, moral, or legal force; to put
      under obligation to do or forbear something.
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            The obliging power of the law is neither founded in,
            nor to be measured by, the rewards and punishments
            annexed to it.                        --South.
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            Religion obliges men to the practice of those
            virtues which conduce to the preservation of our
            health.                               --Tillotson.
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   3. To bind by some favor rendered; to place under a debt;
      hence, to do a favor to; to please; to gratify; to
      accommodate.
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            Thus man, by his own strength, to heaven would soar,
            And would not be obliged to God for more. --Dryden.
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            The gates before it are brass, and the whole much
            obliged to Pope Urban VIII.           --Evelyn.
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            I shall be more obliged to you than I can express.
                                                  --Mrs. E.
                                                  Montagu.
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WordNet (r) 3.0 (2006):

obliged
    adj 1: under a moral obligation to do something [syn: duty-
           bound(p), obliged(p)]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "obliged":
   acknowledging, appreciative, beholden, beholden to, bound, bounden,
   bounden to, cognizant of, committed, compelled, crediting,
   duty-bound, forced, grateful, in duty bound, indebted, indebted to,
   made, much obliged, obligate, obligated, obliged to, pledged,
   required, saddled, sensible, thankful, tied, under obligation