obligatory

a.(法律或道义上)必须的，应尽的

obfuscate, obfuscation, obiter dictum, obituary, object, object glass, object lens, object lesson, object to, object to sth., objection, objectionable, objectionably, objective, objectively, objectivity, objector, objet d'art, oblate, obligate, obligation, oblige, obliged, obliging, obligingly, oblique, oblique angle, obliquely, obliqueness, obliquity, obliterate, obliteration, oblivion, oblivious, obliviousness, oblong, obloquy, obnoxious, obnoxiously, obnoxiousness, oboe,