Whenever order is created,This needs to copy related opportunity line items to order line items.

copyOppLineItems.apxt

/**
* TriggerName : copyOppLineItems
* @description : Whenever order is created,
* 				 This needs to copy related opportunity line items to order line items.
*/
trigger copyOppLineItems on Order (after insert) {	
    
    List<OrderItem> orderItemList = new List<OrderItem>();    
    List<Order> orderList = [SELECT Id, OpportunityId, EffectiveDate, EndDate, IsDeleted, 
                             IsReductionOrder, Name, OrderNumber, OrderReferenceNumber, 
                             OriginalOrderId,TotalAmount FROM Order WHERE Id IN :Trigger.New];
    System.debug(orderList);  // first step, Getting all new order(s) from trigger.new 
    
     
    if(orderList.size() > 0){       
     List<Id> oppIds = copyOppItemHelper.getOppIds(orderList); // Second step, get opportunityId(s) from all those order(s).
        System.debug(oppIds);
        
        if(oppIds.size() > 0) {
          List<OpportunityLineItem> oppItemList = copyOppItemHelper.getOppItemList(oppIds); 
                            // Third step, Get related Opportunity Item Lists from all those Opportunity(s).
             System.debug(oppItemList);
            
            if(oppItemList.size() > 0) {
                orderItemList = copyOppItemHelper.getOrderItemList(orderList, oppItemList);
                    // fourth step, opportunity list items to new order items that match the Criteria. 
                 System.debug(orderItemList);
            }
            
            else{
                System.debug('There are zero number of Opportunity Line Items That matches the Criteria');
            }
            
        }
        
        else{
            System.debug('No opportunities are associted with the new Orders');
        }
    }
    
    else {
        System.debug('Order List is Empty - No New Orders');
    }
    
    
    if(orderItemList.size() > 0) { 
        //Finally, Save the records in to database, and log the errors.
        Database.SaveResult[] srList = Database.insert(orderItemList, false);
        for (Database.SaveResult sr : srList) {
            if (sr.isSuccess()) {
                //  fetching the ids of successful records
                System.debug('Successfully Copied Opportunity Line Item,  ID: ' + sr.getId());
            }
            else 
            {
                // This is for failed records
                for(Database.Error err : sr.getErrors()) {
                    System.debug('Error --> ');  //Printing error message in Debug log
                    System.debug(err.getStatusCode() + ': ' + err.getMessage());
                }
            }
            
        }
    }
    
    else {
        System.debug('No Items to Copy from Opportunity Line Items');
    }
    
}