From Longman Dictionary of Contemporary Englishoffice partyˌoffice ˈparty noun [countable] DLBBOa party, especially one held before Christmas, in the office of a company, government department etc for the people who work there
Examples from the Corpus
office party• We met at an office party almost a year ago.• For a tenner, I snapped up a dinner-jacket for an office party, and gave it back afterwards.• She looked as if she were about to set off for a provincial cocktail party, an office party of female executives.• Like a Mephistco office party, but even less entertaining. 