Problem Description
In my production database Oracle 10.2g while I was adding column to one of my transaction table it fails with ORA-54 error as below.

SQL> alter table student add b number;
alter table student add b number
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified

Description of the Problem
Let's try to produce the problem in our development environment. I have opened two session that connected to database under a schema. In one session, I have created a table and inserted data into it.
SQL> create table a (a number);

Table created.

SQL> insert into a values(1);

1 row created.

I did not yet committed data in session 1. Now in another session whenever I try to any ddl like (alter table, drop table) ORA-00054 will produce.

In another session,
SQL> alter table a add b number;
alter table a add b number
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified


SQL> drop table a;
drop table a
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified

SQL> lock table a in exclusive mode nowait;
lock table a in exclusive mode nowait
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified


Cause of the Problem
Whenever you try to do any structural changes on a table oracle try to lock the table exclusively with NOWAIT option(this is in 10.2g while in 11g you can change the wait timeout). If oracle fails to lock the table exclusively then ORA-00054 will occur.

Solution of the Problem
In 10.2g you are limited to several choices to solve the problem. To avoid it,

-Re run the DDL at a later time when the database become idle.
or,

-Kill the sessions that are preventing the exclusive lock.
or,

-Prevent end user to connect to the database and then run the DDL.

You have different views to see locking information about the table.
1)DBA_BLOCKERS: Displays a session if it is not waiting for a locked object but is holding a lock on an object for which another session is waiting. In our scenario this view will not help.

2)DBA_DDL_LOCKS: It lists all DDL locks held in the database and all outstanding requests for a DDL lock.

3)DBA_DML_LOCKS: It lists all DML locks held in the database and all outstanding requests for a DML lock.
If you query from it in the mode_held field you will see 'row exclusive lock'.
SQL> select mode_held from dba_dml_locks where owner='MAXIMSG';

MODE_HELD
-------------
Row-X (SX)

4)DBA_LOCK: It lists all locks or latches held in the database, and all outstanding requests for a lock or latch.

5)DBA_LOCK_INTERNAL: It displays a row for each lock or latch that is being held, and one row for each outstanding request for a lock or latch.

6)DBA_LOCKS is a synonym for DBA_LOCK.

7)DBA_WAITERS: Shows all the sessions that are waiting for a lock.

8)V$LOCK: It lists the locks currently held by the Oracle Database and outstanding requests for a lock or latch.

9)V$LOCK_ACTIVITY: This view is deprecated.

10)V$LOCKED_OBJECT: This view lists all locks acquired by every transaction on the system.

In order to see locked object query,

SQL> set linesize 130

SQL> set pages 100

SQL> col username       format a20

SQL> col sess_id        format a10

SQL> col object format a25

SQL> col mode_held      format a10

SQL> select     oracle_username || ' (' || s.osuser || ')' username,  

s.sid || ',' || s.serial# sess_id,  

owner || '.' || object_name object,  

object_type,  

decode( l.block,       0, 'Not Blocking',       1, 'Blocking',       2, 'Global') status,  

decode(v.locked_mode, 0, 'None', 1, 'Null',  2, 'Row-S (SS)',  3, 'Row-X (SX)',  4, 'Share',  5, 'S/Row-X (SSX)',   6, 'Exclusive', TO_CHAR(lmode)) mode_held

from       v$locked_object v,  dba_objects d,  v$lock l,  v$session s

where      v.object_id = d.object_id

and        v.object_id = l.id1

and        v.session_id = s.sid

order by oracle_username,  session_id

/

USERNAME             SESS_ID    OBJECT                    OBJECT_TYPE         STATUS       MODE_HELD

-------------------- ---------- ------------------------- ------------------- ------------ ----------

MAXIMSG (A)          142,232    MAXIMSG.A                 TABLE               Not Blocking Row-X (SX)OMS (DBA2\ershad)    143,280   

 OMS.T                     TABLE               Not Blocking Row-X (SX)OMS (DBA2\ershad)    143,280    OMS.T1                   

 TABLE               Not Blocking Row-X (SX)

Here we see there is 3 types of locking. In our case the object is A. And we see the sid=142 and serial#=232 is preventing us from locking the table in exclusive mode. So to achieve our task we can kill the session as below.

SQL> alter system kill session '142, 232';

System altered.

SQL> alter table a add b number;

Table altered.

Now in the first session whenever you try to access error will be generated saying that session has been killed. In the first session.
SQL> commit;
commit
*
ERROR at line 1:
ORA-00028: your session has been killed