Use the Sorting text field in category-editing it is possible to setup a sort-index for categories, as seen in screenshot.
Sorting products is not as straight-forward (though it should made so in future release of OXID).
If a Product belongs to a particular Category, go to the category listing, pull up the category-details, and click on the Sortig tab. Click on Sort Categories, which will open a pop-up with two tables. You can drag and drop the products from Current Sorting into New Sorting in the order you wish the products to sort in this particular category.
When done, do not forget to click on Save new sorting. You can verify if the new sort-order saved on the front-end of your shop!