The S3 API is a really good example of the “OSS only becomes dominant when development slows down” principle. As a friend of mine who has had to support a lot of local blob storage says, “On the gates of hell are emblazoned — S3 compatible.”
That's generally where open standards come from. You document an existing technology and then get independent implementations.
Unix was proprietary technology. POSIX is an open standard.
Even when the standard comes at the same time as the first implementation, it's usually because the first implementer wrote the standard -- there has to be one implementation before there are two.