Auto Update Custom Field on Sales Order

Thanks for posting in the Community, @step7consulting. 

We have custom fields available in QuickBooks Desktop (QBDT). Since you're using a third-party tool to auto-create sales orders, I'd first suggest contacting their support team. They can help confirm if they have the functionality to auto-update the custom field.

If you've verified that there isn't a feature for this, you might want to consider our custom fields functionality in the program. This lets you use a custom field to track your specific customer and item details. I'll walk you through the steps. 

1. Go to Customers from the top menu. 
2. Select Customer Center. 
3. Double-click the employee's name on the list to open the profile. 
4. Go to the Additional Info tab.
5. Choose Define Fields.
6. Name your custom field in the Label column, such as Class. 
7. In the Use for column, check the Cust box. 
8. Click OK. 

The screenshot below shows you the last three steps. For detailed instructions, go through this article: Create and Use Custom Fields. You'll also see the steps on how you can create custom item fields and add it to your sales forms and your reports. 

You can always go to Reports from the top menu. Then, choose Sales and either select Open Sales Orders by Customer or Open Sales Orders by Item report. These two reports allows you to view all your sales orders within a particular date. 

Keep me posted if there's anything else you need. Have a great day, @step7consulting. 

To directly answer your question, sorry you can not get that done in QB.  There are no conditional fields in QB, nor is there scripting for something like that

Maybe your third party utility can do it, as suggested check with them

A very clumsy work around in QB would be to memorize a sales order template for each condition.  If this is the only condition, Class is OOS or not, set up each template and choose the one you need for that order - again very clumsy