Oracle Database Version

Run Below Query in Sql Developer or Toad or any tool you are using.

SELECT * FROM V$VERSION

or

SELECT version FROM V$INSTANCE

or

BEGIN DBMS_OUTPUT.PUT_LINE(DBMS_DB_VERSION.VERSION || '.' || DBMS_DB_VERSION.RELEASE); END;

Result:

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production