Why is ouster the act of ousting and not one who ousts?

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Armenian archbishop calls 4 days of protests, seeks PM's ouster VOA Pronunciation Guide XS SM MD LG

ouster pronunciation Oyster is spelled as After English to Urdu translation of Oyster, If you have issues in pronunciation than you can hear the audio of it in the online

ouster sdk Armenian archbishop calls 4 days of protests, seeks PM's ouster VOA Pronunciation Guide XS SM MD LG