trigger Order3_OrderItemCreate on Order (after insert, before update) {

// ERROR - I believe the problem is READ ONLY product field on OrderItem - put question to 
// Community 6/6/2014 : 
// https://developer.salesforce.com/forums/ForumsMain?id=906F0000000AXWbIAO

    // PULL SETS OF OPPORTUNITY IDS
    Set<id> Opp_Ids = new Set<id>();
    for (Order sc : trigger.new) {
        Opp_Ids.add(sc.Opportunity__c);
    }

    // PULL AN ARRAY OF ALL OPPORTUNITY LINE ITEM RECORDS
    OpportunityLineItem[] OLI_Array = [Select OpportunityId, 
    		Id,
			Quantity,
			UnitPrice,
			PriceBookEntryId
		from OpportunityLineItem where OpportunityId in :Opp_Ids];

    // CREATE A OPPORTUNITYLINEITEM MAP AND FILL IT WITH THE ARRAY
    Map<id, OpportunityLineItem> OLI_Map = new Map<id, OpportunityLineItem>();
    for (OpportunityLineItem olimap : OLI_Array)
        OLI_Map.put(olimap.Id, olimap);

    // Create an asset for each line item on the opportunity     
    OrderItem[] clis = new OrderItem[]{};

	for (Order sc : trigger.new) {
		string opptyId = sc.Opportunity__c;
		
        for (OpportunityLineItem oli : OLI_Array) {
        	system.debug('DEBUG CHECK ID : ' + OLI_Map.get(oli.Id).PriceBookEntryId);
            // IF THE RECORDS ARE FOR THE CURRENT OPPORTUNITY, THEN CREATE OBJECT, BUT DON'T INSERT YET
            if (oli.OpportunityId == opptyId) {
		        OrderItem cli = new OrderItem();
		        cli = new OrderItem();
		        	cli.UnitPrice = OLI_Map.get(oli.Id).UnitPrice;
		        	cli.Quantity = OLI_Map.get(oli.Id).Quantity;
                    cli.PriceBookEntryId = OLI_Map.get(oli.Id).PriceBookEntryId;
                clis.add(cli);
            }
		}
    }
    // INSERT ACTUAL RECORDS
    insert clis;

}