"{!reductionOrderItemNumbersMap[orderItem.originalorderItemId]}"

<apex:page tabStyle="Order"  controller="OrderProductController">
<apex:form >
    <apex:pageBlock>
        <div>
            <apex:pageBlockTable id="orderTable" value="{!orderItems}" var="orderItem" styleClass="list" headerClass="headerRow">
                <apex:column >
                  <apex:facet name="header">
                      <b><u>{!$Label.ORDER_NUMBER}</u></b>
                  </apex:facet>
                  <apex:outputText value="{!orderItem.Order.OrderNumber}"/>
                </apex:column>

                <apex:column >
                  <apex:facet name="header" >
                     <b><u>{!$Label.ORDER_PRODUCT_NUMBER}</u></b>
                  </apex:facet>
                  <apex:outputText value="{!reductionOrderItemNumbersMap}"/>
                </apex:column>

            </apex:pageBlockTable>
        <div/>
    </apex:pageBlock>
</apex:form>
</apex:page>

public with sharing class OrderProductController {
    private Id orderId;
    private List<OrderItem> newOrderItems = new List<OrderItem>();
    private Order reductionOrder = null;
    private Map<Id, OrderItem> reductionOrigOrderItemsMap;
    public Map<Id, String> reductionOrderItemNumbersMap{get;set;}

    public OrderProductController(Id orderId) {
        this.orderId = orderId;

        getRO();
        getOrderItems();
    }

    public Order getRO() {
        if(reductionOrder == null) {
            reductionOrigOrderItemsMap = new Map<Id, OrderItem>();
            reductionOrder = [SELECT ContractId, OrderNumber,Pricebook2Id,Contract.CurrencyIsoCode,
                                     (SELECT Id, Order.Id, Order.ContractId, Order.OrderNumber,
                                             OrderItemNumber,OriginalOrderItemId FROM OrderItems)
                                     FROM Order WHERE Id =: orderId];

            for(OrderItem oi : reductionOrder.OrderItems) {
                reductionOrigOrderItemsMap.put(oi.OriginalOrderItemId, oi);
                System.debug('****reductionOrigOrderItemsMap'+reductionOrigOrderItemsMap);

            }
        }
        return reductionOrder;
    }

    public List<OrderItem> getOrderItems() {
        reductionOrder = getRO();
        reductionOrderItemNumbersMap = new Map<Id, String>();
        System.debug('###reductionOrder:'+reductionOrder);
        if((newOrderItems == null || newOrderItems.size() == 0) && reductionOrder != null) {
            newOrderItems = new List<OrderItem>();
            List<OrderItem> origOrderItems = new List<OrderItem>();
            Set<Id> origOrderItemIds = new Set<Id>();
            if(reductionOrigOrderItemsMap != null && reductionOrigOrderItemsMap.keySet().size() > 0) {
                origOrderItemIds = reductionOrigOrderItemsMap.keySet();
            }
            System.debug('###reductionOrigOrderItemsMap:'+reductionOrigOrderItemsMap);
            System.debug('###origOrderItemIds:'+origOrderItemIds);

            origOrderItems = [SELECT Id, Order.Id, Order.ContractId, Order.OrderNumber, OrderItemNumber,sfbase__AvailableQuantity__c,sfbase__OrderItemEffectiveDate__c,
                                                    PricebookEntry.Pricebook2Id, PricebookEntry.Product2Id
                                                    FROM OrderItem
                                                    WHERE sfbase__AvailableQuantity__c > 0
                                                    AND Order.ContractId =: reductionOrder.ContractId
                                                    AND (Order.StatusCode = 'A' OR Order.StatusCode = 'E')
                                                    AND Id NOT IN : origOrderItemIds
                                                    ORDER BY Order.OrderNumber, sfbase__OrderItemEffectiveDate__c];
            }

            for(OrderItem oi : origOrderItems) {
                reductionOrderItemNumbersMap.put(sglOrderItem.Id,sglOrderItem.OrderItemNumber);
                Double qty = null;
                if(reductionOrigOrderItemsMap.get(oi.Id) != null) {
                    qty = (-1) * reductionOrigOrderItemsMap.get(oi.Id).Quantity;
                } else {
                    continue;
                }

                OrderItem orderItem = new OrderItem(Id = oi.Id,
                                                    OriginalOrderItemId = oi.Id,
                                                    UnitPrice = oi.UnitPrice,
                                                    sfbase__AvailableQuantity__c = oi.sfbase__AvailableQuantity__c,
                                                    Quantity = qty);

                orderItem.Order = oi.Order;
                newOrderItems.add(orderItem);

            }
        System.debug('###reductionOrigOrderItemNumbersMap:'+reductionOrderItemNumbersMap);
        System.debug('###newOrderItems:'+newOrderItems);
        return newOrderItems;
    }
}