There are lots of scripts available on metalink. Here's one.

SELECT r.name "RBS", s.sid, s.serial#, s.username "USER", t.status,
t.cr_get, t.phy_io, t.used_ublk, t.noundo,
substr(s.program, 1, 78) "COMMAND"
FROM sys.v_$session s, sys.v_$transaction t, sys.v_$rollname r
WHERE t.addr = s.taddr
and t.xidusn = r.usn
ORDER BY t.cr_get, t.phy_io
/


Look at used_ublk