Oracle Session Activity

The following script shows the event that is blocking the sessions with high activity

if no rows are displayed it means that the database is working fine with no blocking sessions 

 select

   event,blocking_session,wait_class,
   count( sid)  sessions_waiting
    from
   v$session
where
   blocking_session is not NULL
   group by event,blocking_session,wait_class
order by
   count(1) desc;