public with sharing class OrderController {

    @AuraEnabled(Cacheable=true)
    public static Order_Item__c[] getOrderItems(Id orderId) {
        return [SELECT Id, Qty_S__c, Qty_M__c, Qty_L__c, Price__c,
            Product__r.Name, Product__r.MSRP__c, Product__r.Picture_URL__c
            FROM Order_Item__c WHERE Order__c = :orderId];
    }
}