Hi,
I think that what Nicolas was saying is that if you filter on Categories, it will display all the products of one category, then if you filter on one product, it will display only one product in the listing, so after that filtering on a custom field seems to be a useless step.
But if you filter on a category, then on a custom field, it will display all the products with that specific custom field value.
In your case:
First visitor need to find product in first filter , then he need to choose OS
I think that the product should be a category, so create a filter on a category, then the os can be a custom field value, so create a filter on the desired custom field.
Hope this will help you.