ORA-00054: resource busy and acquire with NOWAIT specified

Solution 1: Step 1: Identify the session which is locking the object
select a.sid, a.serial#
from v$session a, v$locked_object b, dba_objects c 
where b.object_id = c.object_id 
and a.sid = b.session_id
and OBJECT_NAME='EMP';

Step 2: kill that session using
alter system kill session 'sid,serial#'; 

Solution 2:
SQL> alter table emp read only;
Session altered.

SQL> alter table emp add (mobno varchar2(20));
Table altered.


Solution 3 (for 10g):
DECLARE
 MYSQL VARCHAR2(250) := 'alter table emp add (mobno varchar2(15))';
 IN_USE_EXCEPTION EXCEPTION;
 PRAGMA EXCEPTION_INIT(IN_USE_EXCEPTION, -54);
BEGIN
 WHILE TRUE LOOP
  BEGIN
   EXECUTE IMMEDIATE MYSQL;
   EXIT;
  EXCEPTION
   WHEN IN_USE_EXCEPTION THEN 
    NULL;
  END;
  DBMS_LOCK.SLEEP(1);
 END LOOP;
END;


Solution 4: 
SQL> alter session set ddl_lock_timeout = 700;
SQL> alter table emp add (mobile varchar2(15));