Use the word Uhr for "o'clock". Note the position of Uhr in the German phrase. It follows the hour and precedes the minutes, if they are mentioned.
 

Es ist fünf Uhr.	It’s five o'clock.
Es ist fünf Uhr fünfzehn.	It's five fifteen.

German halb means "half." Halb zehn means "halfway to ten" or 9:30. Halb zehn and neun Uhr dreißig are used interchangeably for the same time.
 
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