WordNet records no synonym and no antonym for outage. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 amount of something (as whiskey or oil) lost in storage or transportation
a temporary suspension of operation (as of computers)
“there will be a network outage from 8 to 10 a.m.”
Synonyms, antonyms and the broader and narrower relations for outage come from Princeton University’s WordNet. Full sources and licences.