Hi,

Run the catblock.sql script. This will be available under
\rdbms\admin folder.

After completion of this you can either run utllockt.sql (available on the same folder)
or
select * from dba_blockers ;(lists only the blocking session id)
or
select * from dba_waiters ;(lists all the waiting session, blocking session etc.)

HTH.

Thanks.
Vijay.