Group Name : A
      Ref. No.    In Qty.   Out Qty.    Bal. Qty
      1             2.00       0.00         2.00    
      2             2.00       0.00         4.00
      3             0.00       1.00         3.00
Group Name : B
      Ref. No.    In Qty.   Out Qty.    Bal. Qty
      4             2.00       0.00         2.00    
      5             2.00       0.00         4.00
      6             0.00       1.00         3.00

Group Name : A
      Ref. No.    In Qty.   Out Qty.    Bal. Qty
      1             2.00       0.00         2.00    
      2             2.00       0.00         4.00
      3             0.00       1.00         3.00
Group Name : B
      Ref. No.    In Qty.   Out Qty.    Bal. Qty
      4             2.00       0.00         5.00    
      5             2.00       0.00         7.00
      6             0.00       1.00         6.00

CREATE TABLE mhe_foo(grpno  VARCHAR2(1)
                    ,refno  NUMBER
                    ,inqty  NUMBER
                    ,outqty NUMBER
                    )
/

INSERT INTO mhe_foo VALUES('A', 1, 2.00, 0.00)--    2.00
/    
INSERT INTO mhe_foo VALUES('A', 2, 2.00, 0.00)--    4.00
/
INSERT INTO mhe_foo VALUES('A', 3, 0.00, 1.00)--    3.00
/
INSERT INTO mhe_foo VALUES('B', 4, 2.00, 0.00)--    2.00    
/
INSERT INTO mhe_foo VALUES('B', 5, 2.00, 0.00)--    4.00
/
INSERT INTO mhe_foo VALUES('B', 6, 0.00, 1.00)--    3.00
/
COMMIT
/

SELECT grpno
     , refno
     , inqty
     , outqty
     , SUM(inqty - outqty) OVER ( PARTITION BY grpno ORDER BY grpno, refno) balqty
FROM   mhe_foo
ORDER  BY grpno, refno

SELECT   (' ') sr_no,
         To_date('01-01-06',
                 'dd-mm-yy') voucher_date,
         ('USD') currency,
         (SELECT exchange_rate
          FROM   currency_master
          WHERE  currency_code = 'USD') ex_rate,
         Initcap(('OPENING BALANCE')) ac_name,
         Round((ref_master.opening_rate_usd * currency_master.exchange_rate),
               2) net_usd,
         0,
         ref_master.opening_stock_qty,
         0,
         ref_master.description,
         ' ',
         ref_master.ref_no,
         'OPENING BALANCE' final_desc
FROM     currency_master,
         ref_master
WHERE    currency_master.currency_code = :fcurrency
         AND ref_master.company_id = :fcomp_id & p_query
GROUP BY ref_master.ref_no,
         ref_master.opening_stock_qty,
         ref_master.description,
         ref_master.opening_rate_usd,
         currency_master.exchange_rate
UNION ALL
SELECT   ALL To_char(purchase_master.sr_no) sr_no,
             purchase_master.voucher_date,
             purchase_master.currency,
             purchase_master.ex_rate,
             Initcap(ac_master.ac_name) ac_name,
             Round((purchase_detail.rate_usd * currency_master.exchange_rate / purchase_master.ex_rate),
                   2) net_usd,
             0,
             purchase_detail.quantity,
             0,
             ref_master.description,
             'PU',
             purchase_detail.stock,
             (ac_master.ac_name || ' / ' || purchase_master.invoice_no || ' / ' || ref_master.description) final_desc
FROM     purchase_master,
         ac_master,
         annexure_master,
         purchase_detail,
         currency_master,
         ref_master
WHERE    ref_master.ref_no = purchase_detail.stock
         AND ref_master.ref_group = purchase_detail.stock_group
         AND ref_master.company_id = purchase_master.company_id
         AND ac_master.ac_code = purchase_master.supplier_id
         AND annexure_master.code = purchase_master.annexure_code
         AND annexure_master.code = ac_master.annexure_code
         AND ac_master.company_id = purchase_master.company_id
         AND purchase_master.sr_no = purchase_detail.sr_no
         AND purchase_master.company_id = purchase_detail.company_id
         AND voucher_date BETWEEN :fstart_date
                                  AND :fend_date
         AND currency_master.currency_code = :fcurrency
         AND purchase_master.company_id = :fcomp_id & p_query
GROUP BY purchase_detail.stock,
         purchase_master.sr_no,
         purchase_master.voucher_date,
         purchase_master.currency,
         purchase_master.ex_rate,
         purchase_master.supplier_id,
         ac_master.ac_name,
         purchase_detail.rate_usd,
         currency_master.exchange_rate,
         purchase_master.invoice_no,
         ref_master.description,
         purchase_detail.quantity
UNION ALL
SELECT   ALL To_char(sales_master.sr_no) sr_no,
             sales_master.voucher_date,
             sales_master.currency,
             sales_master.ex_rate,
             Initcap(ac_master.ac_name) ac_name,
             0,
             Round((sales_detail.rate_usd * currency_master.exchange_rate / sales_master.ex_rate),
                   2) net_usd,
             0,
             sales_detail.quantity,
             ref_master.description,
             'SL',
             sales_detail.stock,
             (ac_master.ac_name || ' / ' || sales_master.invoice_no || ' / ' || ref_master.description) final_desc
FROM     sales_master,
         sales_detail,
         salesman_master,
         stock_type,
         ac_master,
         annexure_master,
         currency_master,
         ref_master
WHERE    ref_master.ref_no = sales_detail.stock
         AND ref_master.ref_group = sales_detail.stock_group
         AND sales_master.company_id = ref_master.company_id
         AND sales_master.sr_no = sales_detail.sr_no
         AND sales_master.company_id = sales_detail.company_id
         AND ac_master.ac_code = sales_master.customer_id
         AND annexure_master.code = sales_master.annexure_code
         AND ac_master.company_id = sales_master.company_id
         AND annexure_master.code = ac_master.annexure_code
         AND sales_master.voucher_date BETWEEN :fstart_date
                                               AND :fend_date
         AND currency_master.currency_code = :fcurrency
         AND sales_master.company_id = :fcomp_id & p_query
GROUP BY sales_detail.stock,
         sales_master.sr_no,
         sales_master.voucher_date,
         sales_master.currency,
         sales_master.ex_rate,
         ac_master.ac_name,
         sales_master.invoice_no,
         ref_master.description,
         sales_detail.rate_usd,
         currency_master.exchange_rate,
         sales_detail.quantity

SELECT alias1
     , alias2
     ...
     ...
     , SUM(inqty - outqty) OVER ( PARTITION BY grpno ORDER BY grpno, refno) balqty
FROM   ( your_large_query )