##########################
Current running sessions
##########################
select sid,username,status,event,row_wait_obj#,sql_id,blocking_session,blocking_instance from gv$session where blocking_session is not null;




############################
If its with in < than 2 hours,
###########################
SELECT DISTINCT A.SQL_ID,TO_CHAR(A.SAMPLE_TIME,'YYYY-MM-DD HH24:MI:SS') SAMPLE_TIME,A.BLOCKING_SESSION,D.OBJECT_NAME,S.SQL_TEXT
FROM v$ACTIVE_SESSION_HISTORY A, GV$SQL S, DBA_OBJECTS D
WHERE A.SQL_ID=S.SQL_ID
AND BLOCKING_SESSION IS NOT NULL
AND A.USER_ID <> 0
AND A.CURRENT_OBJ# = D.OBJECT_ID
AND A.SAMPLE_TIME BETWEEN
TO_TIMESTAMP('14-SEP-2017 02:00:00', 'DD-MON.YYYY HH24:MI:SS') AND
TO_TIMESTAMP('14-SEP-2017 12:00:00', 'DD-MON-YYYY HH24:MI:SS')
AND A.EVENT = 'enq: TX - row lock contention'
ORDER BY
SAMPLE_TIME DESC;


############################
If its more than 2 hours 
###########################
SELECT DISTINCT A.SQL_ID,TO_CHAR(A.SAMPLE_TIME,'YYYY-MM-DD HH24:MI:SS') SAMPLE_TIME,A.BLOCKING_SESSION,D.OBJECT_NAME,S.SQL_TEXT
FROM DBA_HIST_ACTIVE_SESS_HISTORY A, GV$SQL S, DBA_OBJECTS D
WHERE A.SQL_ID=S.SQL_ID
AND BLOCKING_SESSION IS NOT NULL
AND A.USER_ID <> 0
AND A.CURRENT_OBJ# = D.OBJECT_ID
AND A.SAMPLE_TIME BETWEEN
TO_TIMESTAMP('14-SEP-2017 02:00:00', 'DD-MON.YYYY HH24:MI:SS') AND
TO_TIMESTAMP('14-SEP-2017 12:00:00', 'DD-MON-YYYY HH24:MI:SS')
AND A.EVENT = 'enq: TX - row lock contention'
ORDER BY
SAMPLE_TIME DESC;