-- Views (use USER_VIEWS or DBA_VIEWS if needed):
SELECT TEXT FROM ALL_VIEWS WHERE upper(VIEW_NAME) LIKE upper('%VIEW_NAME%');
-- Or:
SELECT dbms_metadata.get_ddl('VIEW', 'VIEW_NAME', 'OWNER_NAME') FROM DUAL;

-- Materialized views (use USER_VIEWS or DBA_VIEWS if needed):
SELECT QUERY FROM ALL_MVIEWS WHERE upper(MVIEW_NAME) LIKE upper('%VIEW_NAME%');
-- Or:
SELECT dbms_metadata.get_ddl('MATERIALIZED_VIEW', 'VIEW_NAME', 'OWNER_NAME') 
FROM DUAL;