You have to mention the following properties in the oozie-site.xml file to make Oozie use Oracle: 
    <property>
        <name>oozie.service.JPAService.jdbc.driver</name>
        <value>oracle.jdbc.OracleDriver</value>
    </property>
    <property>
        <name>oozie.service.JPAService.jdbc.url</name>
        <value>jdbc:oracle:thin:@//myhost:1521/oozie</value>
    </property>
    <property>
        <name>oozie.service.JPAService.jdbc.username</name>
        <value>oozie</value>
    </property>
    <property>
        <name>oozie.service.JPAService.jdbc.password</name>
        <value>oozie</value>
    </property>