Historical Blocking Locks

To Investigate Recent Blocking Locks (after the dust settles)

set pagesize 50
set linesize 120
col sql_id format a15
col inst_id format '9'
col sql_text format a50
col module format a10
col blocker_ses format '999999'
col blocker_ser format '999999'

------------------------------------------------------------------
--IN CHRONOLOGICAL ORDER (which is probably what you want anyways)
------------------------------------------------------------------
 SELECT distinct
        a.sql_id ,
        to_char(a.sql_exec_start,'DD-Mon HH24:MI') sql_start,
        a.inst_id,
        a.blocking_session blocker_ses,
        a.blocking_session_serial# blocker_ser,
        a.user_id,
        s.sql_text,
        a.module
 FROM  GV$ACTIVE_SESSION_HISTORY a,
       gv$sql s
 where a.sql_id=s.sql_id
   and blocking_session is not null
   and a.user_id <> 0 --  exclude SYS user
   and a.sample_time > sysdate - 1
 order by sql_start