DBMS_LOGMNR, or "How to see what's in the redo logs"

Contents:
1 Database-Level Supplemental Logging
2 Redo Log File Options
3 Example of Querying V$LOGMNR_CONTENTS
4 End the LogMiner Session

Database-Level Supplemental Logging:

ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;

Redo Log File Options

Here you will tell you LOGMNR whate logfiles to look into. It can be done:

Automatically. Just define the time range you want to observe and start logmnr.

ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY HH24:MI:SS';

EXECUTE DBMS_LOGMNR.START_LOGMNR( STARTTIME => '01-Jan-2003 08:30:00', ENDTIME => '01-Jan-2003 08:45:00', OPTIONS =>
DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG + DBMS_LOGMNR.CONTINUOUS_MINE);

Manually. You need to manually add every logfile you're interested in and then start logmnr.

EXECUTE DBMS_LOGMNR.ADD_LOGFILE( LOGFILENAME => '/oracle/logs/log1.f', OPTIONS => DBMS_LOGMNR.NEW);

EXECUTE DBMS_LOGMNR.ADD_LOGFILE( LOGFILENAME => '/oracle/logs/log.f', OPTIONS => DBMS_LOGMNR.ADDFILE);

EXECUTE DBMS_LOGMNR.START_LOGMNR(OPTIONS => DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG);

Example of Querying V$LOGMNR_CONTENTS

SELECT OPERATION, SQL_REDO, SQL_UNDO
  FROM V$LOGMNR_CONTENTS
  WHERE SEG_OWNER = 'OE' AND SEG_NAME = 'ORDERS' AND
  OPERATION = 'DELETE' AND USERNAME = 'RON';

End the LogMiner Session

EXECUTE DBMS_LOGMNR.END_LOGMNR;

Note: Everything is related to your session, so if you got disconnected, start again

Source: http://download.oracle.com/docs/cd/B28359_01/server.111/b28319/logminer.htm#i1015913