1) user A locks row 1 in table X
2) user B locks row 33 in table Z
3) user A tries to lock row 33 in table Z - his session is now hanging, waiting for the lock gained by user B in step 2
4) user B tries to lock row 1 in table X - his session is now hanging, waiting for the lock gained by user A in step 1.

SELECT DECODE(request,0,'Holder: ','Waiter: ')||sid sess, 
id1, id2, lmode, request, type
FROM V$LOCK
WHERE (id1, id2, type) IN
(SELECT id1, id2, type FROM V$LOCK WHERE request>0)
ORDER BY id1, request
/