dbms_aw.eval_number

INSERT INTO bom
SELECT 'A','B',3 FROM dual UNION ALL
SELECT 'A','C',2 FROM dual UNION ALL
SELECT 'A','D',4 FROM dual UNION ALL
SELECT 'B','E',2 FROM dual UNION ALL
SELECT 'B','F',3 FROM dual UNION ALL
SELECT 'D','G',6 FROM dual UNION ALL
SELECT 'D','H',5 FROM dual UNION ALL
SELECT 'E','I',3 FROM dual

SELECT part_no,
qty,
DBMS_AW.eval_number(substr(sys_connect_by_path(qty, '*'),2)) AS sum_qty
FROM bom
START WITH item_no = 'A'
CONNECT BY PRIOR part_no = item_no
ORDER SIBLINGS BY 1,2;

PART_NO           QTY    SUM_QTY
---------- ---------- ----------
B                   3          3
E                   2          6
I                   3         18
F                   3          9
C                   2          2
D                   4          4
G                   6         24
H                   5         20
8 rows selected