If you answer at the top, it's easier to read :)

>>I'm getting the error on the last line in the WHERE clasue:
>>
>>

>>USE PKDEMO5
>>GO
>>SET ANSI_NULLS ON
>>GO
>>SET QUOTED_IDENTIFIER ON
>>GO
>>EXEC sp_DropProc 'sp_GetBlanketSalesOrders'
>>GO
>>
>>CREATE PROCEDURE sp_GetBlanketSalesOrders
>>	@ProgMgr		VARCHAR(25) = NULL,		-- Program Manager
>>	@CU_KEY			VARCHAR(12)	= NULL,		-- Customer
>>	@IM_KEY			VARCHAR(30)	= NULL,		-- Item key
>>	@Outstanding	INT			= NULL		-- If 1, only include records where OpenQty <> 0
>>
>>AS
>>
>>	SELECT	CI.TEAM AS ProgramMgr, 
>>			OH.OH_CUKEY AS Customer, 
>>			MOC.MOC_PART_NO AS Item, 
>>			IM.IM_REV AS Rev, 
>>			OH.OH_CUST_PO_NUM AS CustPONum, 
>>			OH.OH_NUMBER AS SPNum, 
>>			MOC.MOC_REF_NO AS SOLine, 
>>			MOC.MOC_USER_11 AS OrigBlanketQty, 
>>			ISNULL(v_BlanketSODeliveryQtyByLine.BookedQty, 0)AS QtyOrdered, 
>>			MOC_USER_11 - ISNULL(v_BlanketSODeliveryQtyByLine.BookedQty, 0) AS RemainingBlanketQty, 
>>			CASE WHEN MOC_USER_21 = '1901-01-01' THEN NULL ELSE MOC_USER_21 END AS BlanketExpDate,
>>			MOC.MOC_PRICE AS LinePrice, 
>>			RQ.RQ_DATE_REQ AS RequiredDate, 
>>			RQ.RQ_DATE_PROM AS PromiseDate, 
>>			RQ.RQ_QTY_REQ AS RequiredQty, 
>>			RQ.RQ_QTY_ISS AS ShippedQty, 
>>			CASE WHEN RQ_STATUS = 'C' OR RQ_QTY_ISS > RQ_QTY_REQ THEN 0 ELSE RQ_QTY_REQ - RQ_QTY_ISS END AS OpenQty
>>		FROM OH 
>>		INNER JOIN MOC ON OH.OH_NUMBER = MOC.MOC_OHNUMBER
>>		LEFT JOIN RQ ON (MOC.MOC_OHNUMBER = RQ.RQ_MOCNUMBER) AND 
>>						(MOC.MOC_REF_NO = RQ.RQ_MOCREFNO) 
>>		LEFT JOIN v_BlanketSODeliveryQtyByLine ON (MOC.MOC_OHNUMBER = v_BlanketSODeliveryQtyByLine.RQ_MOCNUMBER) AND 
>>												  (MOC.MOC_REF_NO = v_BlanketSODeliveryQtyByLine.RQ_MOCREFNO) 
>>		LEFT JOIN IM ON MOC.MOC_PART_NO = IM.IM_KEY 
>>		LEFT JOIN ONCORE_SM.dbo.NV_CUST_INFO CI ON IM.IM_CATALOG = CI.CUCODE
>>		WHERE	((MOC.MOC_USER_11 <> 0) AND (MOC.MOC_STATUS <> 4)) AND
>>	
>>				(@ProgMgr IS NULL OR CI.Team = @ProgMgr) AND
>>				(@CU_KEY IS NULL OR OH.OH_CUKEY = @CU_KEY) AND
>>				(@IM_KEY IS NULL OR IM.IM_KEY = @IM_KEY) AND
>>				(@Outstanding IS NULL OR (OpenQty <> 0))
>>		ORDER BY CI.TEAM, OH.OH_CUKEY, MOC.MOC_PART_NO, OH.OH_NUMBER, MOC.MOC_REF_NO, RQ.RQ_DATE_REQ, RQ.RQ_DATE_PROM
>>

>>
>>OpenQty is defined as the last ite in the field list
>
>You can't use a field that is defined in the main query in WHERE clause.
>That field didn't exists yet.
>You should use the same expression in the WHERE or use the whole query as a derived table and then put WHERE.