That's a standard for a file format, but that doesn't necessarily mean it is a free/open format. I don't think the FSF, for instance, considers it open (maybe because of patent issues). I'll leave that for you to decide, but just mean to suggest a standard doesn't automatically mean free/open.

Interestingly, a section in the wiki article linked mentioned the standard proposal was controversial because ODF already existed (and ODF was considered less complicated as a specification).

Nevertheless, good point. It depends on what you mean by "open".