You can do this within the workflow designer, you don't need to change the database.
Ensure "post form on change" is set on the operating system field.
The form will then reload.
Ensure the defaults and/or list options for the second drop down are populated by a dynamic model.
You can put the whole lot in a dynamic AJAX panel if you want to avoid the flickering.
There are a couple of videos detailing this setup on www.workflowswat.com
Hope this helps a little.
Phil