Antonyms of adj obligatory

2 senses of obligatory

Sense 1
obligatory (vs. optional) -- (morally or legally constraining or binding; "attendance is obligatory"; "an obligatory contribution")

optional (vs. obligatory) -- (possible but not necessary; left to personal choice)
        => elective -- (not compulsory; "elective surgery"; "an elective course of study")
        => ex gratia -- (as a favor; not compelled by legal right; "ex gratia payments made to nonstriking workers")
        => facultative -- (not compulsory; "facultative courses in the sciences")
        => nonmandatory, nonobligatory -- (not required by rule or law)

Sense 2
obligatory -- (required by obligation or compulsion or convention; "he made all the obligatory apologies")

INDIRECT (VIA necessary) -> unnecessary, unneeded -- (not necessary)

Similarity of adj obligatory

2 senses of obligatory

Sense 1
obligatory (vs. optional) -- (morally or legally constraining or binding; "attendance is obligatory"; "an obligatory contribution")
       => bounden -- (morally obligatory; "my bounden duty")
       => compulsory, mandatory, required -- (required by rule; "in most schools physical education is compulsory"; "attendance is mandatory"; "required reading")
       => de rigueur -- (required by etiquette or usage or fashion; "instruction as to when and where a silk hat is de rigueur")
       => imposed -- (set forth authoritatively as obligatory; "the imposed taxation"; "rules imposed by society")
       => incumbent on (predicate) -- (morally binding or necessary; "it is incumbent on me to attend")
       => indispensable -- (unavoidable; "the routine but indispensable ceremonies of state")
       => prerequisite -- (required as a prior condition or course of study)
          Also See-> necessary#1

Sense 2
obligatory -- (required by obligation or compulsion or convention; "he made all the obligatory apologies")
       => necessary (vs. unnecessary) -- (absolutely essential)