top 5 sessions that were using the most CPU resources [or waiting for resources] within the last 15 minutes

SELECT * FROM
(
SELECT s.username, s.module, s.sid, s.serial#, count(*)
FROM v$active_session_history h, v$session s
WHERE h.session_id = s.sid
AND h.session_serial# = s.serial#
AND session_state= 'ON CPU' AND
sample_time > sysdate - interval '15' minute
GROUP BY s.username, s.module, s.sid, s.serial#
ORDER BY count(*) desc
)
where rownum <= 5;

If you wanted to see the sessions that are waiting for
resources, you could issue the same query as previously, with a SESSION_STATE of WAITING