In many comments I have read, people, especially from the FOSS community, have stated that OOXML is not really an open standard and that Microsoft controls the format.
The basics of the openness of a standard in the Intellectual Property (IP) rights. This breaks down into copyrights granted by rights to the author of a work and the patent rights which are granted to patentholders.
For OOXML is the copyrights are now in the hands of Ecma international which created and published the standard. Microsoft contributed to this but has no copyrights to the Ecma standard.
Ecma makes all it's standards available for free:
"Ecma Standards are made available to all interested persons or organizations, free of charge and copyright, in printed form and, as files in Acrobat ® PDF format."
So on the copyright side everything is covered and the standard is as open as it can be. So how about the patent rights. As a standard that originates from technology created by Microsoft the most likely party to have patents that are relevant to OOXML will be Microsoft. To make sure that the standard is not hampered by the burden of implementation which conflicts these possible patents Micrsoft has added a covenant not to sue and later added the Ecma standard to the covered formats of it's Open Specification Promise.
"Microsoft irrevocably promises not to assert any Microsoft Necessary Claims against you for making, using, selling, offering for sale, importing or distributing any implementation to the extent it conforms to a Covered Specification"
In software patents in general patent claims are the methods or system that implement an invention. So basically Microsoft cannot use it patent claim against anyone that requires their patented methods or systems to implements the OOXML format even for commercial use.
Opponent of OOXML has tried to raise the question of what it means that an implementation conforms to a specification but actually OOXML contains a section which clearly states what a conforming implementation is. Describing conformance is of course a matter for Ecma that controls the standard and is not up to Microsoft.
Another point that is often made is that Microsoft's release of its patent claim are not given for future version of the standard where it seems that Sun which has used a similar covenant not to sue on patent claims related to ODF has stated it to be for future versions as well. As the standard no longer belongs to Microsoft it would be very hard to make a statement about future versions of the standard unless they could control what exactly would be in the future versions.
But how about Sun then ?
Strangely enough Sun has made sure that it controls what is in next versions of the ODF standard. Sun's covenant not to sue hold a strange provision that makes sure that for quite a while to come their control over OpenDocument development is secured. Their CNS is limited to: "any subsequent version thereof ("OpenDocument Implementation") in which development Sun participates to the point of incurring an obligation". So Sun's covenant only applies to future version if they participate in development. So if Sun's does not like the development of ODF it can hold up the development of the standard until there is certainty that it does not violate any of sun's patents.
This is quite a big deal. Let's say that Microsoft were to start using ODF (as unlikely as it seems atm) and joined the OASIS TC to help adding an Office database format to ODF. But then Oracle buys up Sun and all it patents and they decide that they don't like a database format added to ODF. Then they could severely block any development and mayby halt the ODF development for years if not alltogehter. This makes it extremely unlikely that Microsoft will give full support to ODF while Sun still hold control over the development.
Because MS has not releases it's claims for future version it has similar control over OOXML as Sun does over ODF. However there is a big difference. Microsoft needs development of the Office format as it is vital to it's core business unlike for instance Sun or IBM.
They need a certain amount of control over the Office format development as if they did not have such control then their competitors would find it easy enough to stifle further development of MS Office technologies and make it easier to catch up.
For Microsoft to not develop newer versions of OOXML or even going back to a closed format in MS Office would be like shooting themselves if the foot.
To conclude I would say that OOXML is an open standard in almost the same way that ODF is open.
The advantage of ODF might be that it will have a broader group of development support (incl. Sun of course) whereas OOXML has the advantage that they have the market leader supporting any new development which can therefore be fairly quick and implementation of a new version can be very rapidly expanded to a wide customerbase which makes it interesting for commercial support.