Check Version of Oracle Database

SELECT * FROM v$version;

OR 

SELECT version FROM V$INSTANCE;

OR

SELECT * FROM v$version WHERE banner LIKE 'Oracle%';

Check the component version of Oracle

select * from PRODUCT_COMPONENT_VERSION;

OR 

Check the dba_registry component:
col comp_id for a10
col version for a11
col status for a10
col comp_name for a37
select comp_id,comp_name,version,status from dba_registry;

Check the version of SQLPLUS

sqlplus -v