occupied has 2 distinct synonyms spread across 2 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is unoccupied.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
held or filled or in use
“she keeps her time well occupied”
seized and controlled as by military invasion
“the occupied countries of Europe”
resided in; having tenants
“not all the occupied (or tenanted) apartments were well kept up”
Synonyms, antonyms and the broader and narrower relations for occupied come from Princeton University’s WordNet. Full sources and licences.
having ones attention or mind or energy engaged
“she keeps herself fully occupied with volunteer activities”