Find the list of objects present in Oracle Database.

DBA_ refer to all objects in database, ALL_ refer to which object having access privilege’s to user plus own created objects, USER_ refer to only own object created you can see.

SELECT * FROM dba_objects;
OR
SELECT * FROM ALL_OBJECTS;
OR
SELECT * FROM USER_OBJECTS;

Find the objects present in particular schema in Oracle

SELECT * FROM dba_objectw where owner = 'schema_name';

List all the dictionary views:

SELECT * FROM dict;

Find the list of all tables present in Oracle Database

Select * from dba_tables;
OR
SELECT * FROM ALL_TABLES:
OR
SELECT * FROM USER_TABLES;

List of all tables columns in Oracle Database

SELECT * FROM all_tab_columns
 WHERE table_name = :tname;