Должен ли выбранный o.id также вводиться в групповой оговорке? Я думаю так.
select Distinct o.id as ID,
o.orderno as "order",
a.itemno as "Item Number",
sum(od.total_qty_ord/a.pallet_ptsper) as "Total Pallets"
From Orders o,
ord_detail od,
arinvt a
where o.id = od.orders_id (+)
and a.id (+) = od.arinvt_id
and o.orderno = '1323-PASO'
group by o.order, a.itemno, o.id ---> include the o.id
order by ID