sObject type ‘OrderItem’ is not supported.

Here’s an error you probably spent WAY too much time trying to figure out.

sObject type ‘OrderItem’ is not supported. If you are attempting to use a custom object, be sure to append the ‘__c’ after the entity name. Please reference your WSDL or the describe call for the appropriate names

Answer: It’s read permission on Pricebook2. Go turn that on.

Thanks to Tsenko Aleksiev for the “research”!