Here is a unified query that should help:
SELECT 'User/Schema' match_type,
username
FROM dba_users -- if you have access to dba_users uses all_users instead
WHERE UPPER(username) LIKE '%THE_STRING%'
UNION
SELECT object_type,
owner || '.' || object_name object_name
FROM dba_objects -- or all_objects if no access to dba_objects
WHERE UPPER ( owner || '.' || object_name ) LIKE '%THE_STRING%'
ORDER BY 1, 2
/
Note, the dba_
views will show everything (that matches), the all_
views will only show matches for objects that you have access to.