If you join OITW in, you will get multiple rows because onhand is for each warehouse.
You may try:
SELECT T4.[Code] as 'Raw material Name', T2.ItemName, SUM(T4.[Quantity]) as 'BOMQTY', SUM((T1.[OpenQty]*T4.[Quantity])) as 'TOTALQTY', T2.Onhand
FROM ORDR T0
INNER JOIN RDR1 T1 ON T0.[DocEntry] = T1.[DocEntry]
INNER JOIN OITT T3 ON T3.[ItemCode] = T1.[Code]
INNER JOIN ITT1 T4 ON T3.[Code] = T4.[Father]
INNER JOIN OITM T2 ON T2.[ItemCode] = T4.[Code]
WHERE T1.[LineStatus] ='o'
GROUP BY T4.[Code], T2.ItemName, T2.Onhand
