User session blocking other sessions

To see what sessions are waiting on a lock to be released, issue the following SQL statement:

SELECT sid,id1 as object_id
FROM v$lock WHERE type='TM' AND request<>0;

If a session is waiting for a lock to be released, you will see the OBJECT_ID of the object the session is waiting for. To see who is holding a lock on that object, issue the following query:

SELECT object_id,owner,object_name,username
FROM v$session s, v$locked_object l, dba_objects o WHERE s.sid=l.session_id AND l.object_id=o.object_id ORDER BY 1;

Look for the object_id in the second query's output to see which session(s) are holding the lock.