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.

If the license is updated in the future, you will receive a notification via DriveThruRPG.

Also of interest is this blog post that describes in simple terms how the third party license works alongside the Open Game License that publishers may also optionally use.