The OED project is open source under the Mozilla Public License (MPL) which is MPL 2.0 as of 2017. This is a legal framework used by many projects to guarantee that software will be forever available and free to everyone. So, if you want to use OED, rest assured that it is completely free and will remain that way forever. The MPL places requirements on anyone who makes modifications to the software. While you should read up on the details if you want to edit the software (MPL FAQ, MPL license and other resources on the web), it does require you to, in general, share your work back with the OED project. The idea is everyone benefits from the free software and sharing is the best way of enabling this.