Hey,
The following two properties are required in Hadoop core-site.xml:
 <property>
    <name>hadoop.proxyuser.[OOZIE_SERVER_USER].hosts</name>
    <value>[OOZIE_SERVER_HOSTNAME]</value>
  </property>
  <property>
    <name>hadoop.proxyuser.[OOZIE_SERVER_USER].groups</name>
    <value>[USER_GROUPS_THAT_ALLOW_IMPERSONATION]</value>
  </property>
Replace the capital letter sections with specific values and then restart Hadoop.