Old-School Essentials Third Party License
A free of charge and royalty free license allowing third party publishers to use a “Designed for use with Old-School Essentials” logo on commercial or non-commercial products.
To download the license, add this free product to your cart and check out. If the license is updated in the future, you will receive a notification.
Also of interest is this blog post that describes in simple terms how the third party license and the Open Game License work.