observation has 5 distinct synonyms spread across 3 of its 5 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
the act of making and recording a measurement
the act of observing; taking a patient look
a remark expressing careful consideration
facts learned by observing
“he reported his observations to the mayor”
the act of noticing or paying attention
“he escaped the notice of the police”
Synonyms, antonyms and the broader and narrower relations for observation come from Princeton University’s WordNet. Full sources and licences.