Check Oracle High Redo Generation

Step 1: Get the segment that experienced the most changes 
during  the period
SELECT to_char(begin_interval_time,'YYYY_MM_DD HH24:MI') snap_time,
        dhsso.object_name,
        sum(db_block_changes_delta)
  FROM dba_hist_seg_stat dhss,
         dba_hist_seg_stat_obj dhsso,
         dba_hist_snapshot dhs
  WHERE dhs.snap_id = dhss.snap_id
    AND dhs.instance_number = dhss.instance_number
    AND dhss.obj# = dhsso.obj#
    AND dhss.dataobj# = dhsso.dataobj#
    AND begin_interval_time
BETWEEN to_date('2008_05_02 12','YYYY_MM_DD HH24')
       AND to_date('2008_05_04 12','YYYY_MM_DD HH24')
  GROUP BY to_char(begin_interval_time,'YYYY_MM_DD HH24:MI'),
           dhsso.object_name
 
Step 2: Run the following query to examine the changes during 
the period to what they were historically.
SELECT to_char(begin_interval_time,'YYYY_MM_DD HH24:MI') snap_time,
        sum(db_block_changes_delta)
  FROM dba_hist_seg_stat dhss,
         dba_hist_seg_stat_obj dhsso,
         dba_hist_snapshot dhs
  WHERE dhs.snap_id = dhss.snap_id
    AND dhs.instance_number = dhss.instance_number
    AND dhss.obj# = dhsso.obj#
    AND dhss.dataobj# = dhsso.dataobj#
    AND dhsso.object_name = 'OT_PENDING'
  GROUP BY to_char(begin_interval_time,'YYYY_MM_DD HH24:MI')

 Step 3: Gather all SQL statements that were run during the 
period that referenced the table, using what is below
SELECT to_char(begin_interval_time,'YYYY_MM_DD HH24:MI'),
         dbms_lob.substr(sql_text,4000,1),
         dhss.instance_number,
         dhss.sql_id,executions_delta,rows_processed_delta
  FROM dba_hist_sqlstat dhss,
         dba_hist_snapshot dhs,
         dba_hist_sqltext dhst
  WHERE upper(dhst.sql_text) LIKE '%OT_PENDING%'
    AND dhss.snap_id=dhs.snap_id
    AND dhss.instance_Number=dhs.instance_number
    AND dhss.sql_id = dhst.sql_id

 Step 4: Take the SQL_ID for the statement and searched 
through DBA_HIST_ACTIVE_SESS_HISTORY, using what is below
SELECT instance_number, to_char(sample_time,'yyyy_mm_dd hh24:mi:ss'),
         user_id,
         program
  FROM dba_hist_active_sess_history
  WHERE sql_id = 'bxjz5d4p6x02u'
    AND snap_id BETWEEN 43212 AND 43220

One of the best document to check the high generation of redo:
http://appcrawler.com/wordpress/2009/04/15/who-is-generating-all-the-redo/