In Oracle PL/SQL, OPAQUE refers to the abstract data types whose internal representation are maintained by Oracle. The opaque type available in Oracle is XMLType. The XMLType data is stored as series of bytes and Oracle does not expose this internal representation. There are separate methods provided by Oracle to access the OPAQUE data.
A table named XMLTEST of XMLType is created. A XML document is inserted as a whole (as a CLOB), whose internal representation is not known.
SQL> CREATE TABLE XMLTEST OF XMLTYPE;
SQL> INSERT INTO XMLTEST values
1 row inserted.
FROM xmltest i,
EXTRACT(i.object_value, '/Business/Location/Service/Basics/Attribute[Name="id"]'))) s