I am trying to create an orderitem from visualforce page and I am getting an error 

> Attempt to de-reference a null object


 while assigning pricebookentry.priduct2Id.Here is my part of code where it is failing.

     for(OrderItem oi:selectedOItems){
            OrderItem o = new OrderItem();
             o.PricebookEntry.Product2Id = oi.PricebookEntry.Product2Id; //error on this line
             o.Quantity = oi.Quantity;
             o.unitPrice = oi.UnitPrice;
             o.orderId = oId; 
             oItemsToInsert.add(o); 
          }

How should I assign product to OrderItem ?