Antonyms of adj optional

1 sense of optional

Sense 1
optional (vs. obligatory) -- (possible but not necessary; left to personal choice)

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)

Similarity of adj optional

1 sense of optional

Sense 1
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)