Original BSD license, OpenBSD is free for use

In keeping with the spirit of the original BSD license, OpenBSD is free for use in any way by anyone. You can use it in any tool you like, on any computer, for any purpose. Most of today's free software is licensed under terms that require distributors of software to return any changes back to the project owner. OpenBSD doesn't come with even that minor requirement. You can take OpenBSD, modify it, and embed it in refrigerators that order replacement food over the Internet, without ever paying the developers a dime. [1]

OpenBSD is perhaps the freest of the free operating systems. Like every other free UNIX-like operating system, the source code tree inherited from OpenBSD originally contained a wide variety of programs that shipped under conditional licenses. Some were free for non-commercial use; some were free if you changed the name once you made a change to the code; others had a variety of obscure licensing terms, such as indemnifying a third party against lawsuits. These have been either ripped out or replaced with freely licensed alternatives.

Theo de Raadt said on a mailing list during a discussion of licensing terms:

     We know what a free license should say.

        It should say

      * Copyright foo

      * I give up my rights and permit others to:

                      distribute

                      sell

                      give

                      modify

                      use

      * I retain the right to be known as the author/owner

      When it says something else, ask this:

      * - is it 100% guaranteed fluff which cannot ever affect anyone?

      * - is it giving away even more rights (the author right)?

      If not, then it must be giving someone more rights, or by the same token -

      taking more rights away from someone else!

      Then it is _less_ free than our requirements state!

The OpenBSD Project does a lot of work to guarantee that its licensing is as stringently free as its code is correct.