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>