List<Product2> listOfProducts = [SELECT Id, Name, Description, IsActive, Parent_Product__c, Account__c ,
                                   (SELECT Id, Product2Id, Quantity FROM OrderItem) FROM Product2 
                                  WHERE Account__c = 'XYZ'];

ERROR at Row:2:Column:74
Didn't understand relationship 'OrderItem' in FROM part of query call. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name. Please reference your WSDL or the describe call for the appropriate names.

List<Product2> listOfProducts = [SELECT Id, Name, Description, IsActive, Parent_Product__c, Account__c ,
                                   (SELECT Id, Product2Id, Quantity FROM OrderItem) FROM Product2 
                                  WHERE Account__c = 'XYZ'];

ERROR at Row:2:Column:74
Didn't understand relationship 'OrderItem' in FROM part of query call. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name. Please reference your WSDL or the describe call for the appropriate names.