LibreOffice blasts 'fake open source' OnlyOffice for working with Microsoft to lock users in
(blog.documentfoundation.org)
or in what way the format remains proprietary.
Most of it is proprietary extensions. There's a whole wikipedia article over the drama.
Stallman quote:
Microsoft corrupted many members of ISO in order to win approval for its phony 'open' document format, OOXML. This was so governments that keep their documents in a Microsoft-only format can pretend that they are using 'open standards.' The government of South Africa has filed an appeal against the decision, citing the irregularities in the process.
And FSFE' stance on it.
Edit: moved it in a separate comment.