Русско-англо-русский юридический словарь

the Supplier shall be entitled but not obliged to charge the BBC interest on the overdue amount from the due date for payment up to the date of payment, ... (bbc.co.uk)

In speaking of a debt, ‘outstanding’ simply means that it hasn’t been paid yet, and therefore has to be paid at some future moment. ‘Overdue’ means that it ought to have been paid already, and that the creditor (=the person it is owed to) has the right to complain, take the debtor to court and even demand extra interest. (Quora)