The Collaborative International Dictionary of English v.0.48:

Obligatory \Ob"li*ga*to*ry\, a. [L. obligatorius: cf. F.
   obligatoire.]
   Binding in law or conscience; imposing duty or obligation;
   requiring performance or forbearance of some act; -- often
   followed by on or upon; as, obedience is obligatory on a
   soldier.
   [1913 Webster]

         As long as the law is obligatory, so long our obedience
         is due.                                  --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

obligatory
    adj 1: morally or legally constraining or binding; "attendance
           is obligatory"; "an obligatory contribution" [ant:
           optional]
    2: required by obligation or compulsion or convention; "he made
       all the obligatory apologies"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "obligatory":
   absolute, binding, choiceless, compulsory, conclusive, de rigueur,
   decisive, decretory, demanded, dictated, entailed, essential,
   exigent, final, hard-and-fast, imperative, imperious, importunate,
   imposed, incumbent, indispensable, inevitable, involuntary,
   irrevocable, mandated, mandatory, must, necessary, necessitous,
   peremptory, prescript, prescriptive, required, requisite, ultimate,
   urgent, without appeal, without choice