what is an oracle sql if statement?

I would suggest:

SELECT COALESCE(O.SALESMAN_ID, 0) as SALESMAN_ID, SUM(OI.UNIT_PRICE * QUANTITY)
FROM ORDERS O JOIN
     ORDER_ITEMS OI
     ON o.ORDER_ID = OI.ORDER_ID. -- guessing at the relationship
GROUP BY COALESCE(O.SALESMAN_ID, 0)
ORDER BY COALESCE(O.SALESMAN_ID, 0);

Your query as written would produce non-sensical results. Always use proper, explicit, standard, readable JOIN syntax. Never use commas in the FROM clause.