How should I store a GUID in Oracle?

CREATE table test (testguid RAW(16) default SYS_GUID() ) 

This blog studied the relative performance.

As others have stated, there is a performance hit using GUIDs compared to numeric sequences. That said, there is a function named "SYS_GUID()" available since Oracle 8i that provides the raw equivalent:

SQL> SELECT SYS_GUID() FROM DUAL;

SYS_GUID()
--------------------------------
248AACE7F7DE424E8B9E1F31A9F101D5

A function could be created to return a formatted GUID:

CREATE OR REPLACE FUNCTION GET_FORMATTED_GUID RETURN VARCHAR2 IS guid VARCHAR2(38) ;
BEGIN
    SELECT SYS_GUID() INTO guid FROM DUAL ;
    
    guid :=
        '{' || SUBSTR(guid,  1, 8) ||
        '-' || SUBSTR(guid,  9, 4) ||
        '-' || SUBSTR(guid, 13, 4) ||
        '-' || SUBSTR(guid, 17, 4) ||
        '-' || SUBSTR(guid, 21) || '}' ;

    RETURN guid ;
END GET_FORMATTED_GUID ;
/

Thus returning an interchangeable string:

SQL> SELECT GET_FORMATTED_GUID() FROM DUAL ;

GET_FORMATTED_GUID()
--------------------------------------
{15417950-9197-4ADD-BD49-BA043F262180}

A note of caution should be made that some Oracle platforms return similar but still unique values of GUIDs as noted by Steven Feuerstein.

Update 11/3/2020: With 10g, Oracle added support for regular expression functions which means the concatenation can be simplified using the REGEXP_REPLACE() function.

REGEXP_REPLACE(
    SYS_GUID(),
    '([0-9A-F]{8})([0-9A-F]{4})([0-9A-F]{4})([0-9A-F]{4})([0-9A-F]{12})',
    '{\1-\2-\3-\4-\5}'
)

The expression breaks out the string value returned by SYS_GUID() into 5 groups of hexadecimal values and rebuilds it, inserting a "-" between each group.