In Odoo , There is no way to add discount on whole purchase order, so to solve the problem we have made one customization solution in odoo for purchase to add discount by fixed and percentage way.
By installing this module, you will find a below fields in Purchase order view.
you can apply a global discount in odoo by two way.
you will select your discount method and set your discount in discount fields. so relevant discount will apply on your Purchase order total with – discount field (Discount Method-Fixed in odoo )
For more information contact us on firstname.lastname@example.org