It means 'Original Poster', and is pretty common in for example Usenet (where you have a thread starting potentially with an anonymous question).
For a complete list of Stack Exchange terms, see Stack Exchange Glossary - Dictionary of Commonly-Used Terms.
I don't consider this an issue, since people who see that message are already on this site. The suggestion is not "Go to TeX.SX" or "Visit TeX.SX".
As an aside: For those posting
Welcome to TeX.SX!
verbatim as a comment, they could consider changing this to
Welcome to TeX.SE!
as the .SE suffix is more commonly used to represent the Stack Exchange ...