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oppression (n)
oppressive (adj)
oppressively (adv)
oppressiveness (n)
oppressor (n)
opprobrious (adj)
opprobriously (adv)
opprobriousness (n)
opprobrium (n)
opt (v)
opt out (v)
optical (adj)
optical illusion (n)
optical instrument (types of)
optical microscope (n)
optimal (adj)
optimism (n)
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optional (adj)
Synonyms: elective, noncompulsory, voluntary, discretionary, possible, uncompelled, free

Antonym: compulsory
Encarta® World English Dictionary [World English Edition] © & (P)2009 Microsoft Corporation. All rights reserved. Developed for Microsoft by Bloomsbury Publishing Plc.
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