Guys, .docx, .xlsx, .pptx, etc, are not Open Document Format (ODF) files, they are Microsoft's "Office Open XML" formats. Note that it's "Office Open XML", as in MS Office 2007, the formats are described (at least by MS) as being open, and they use XML. It's not a reference to the "Open Office" suite, which uses ODF (.odt, .ods, .odp, etc.) as its native file format.
The likely reason for your confusion is that the new MS Office 2007 formats are not readable by MS Office 2003 or earlier without a special compatibility pack. OTOH, Open Office (and Apple's iWork suite, and Google Docs) will open MS's 2007 file formats without issue, which is probably why that's the default on your system. To the best of my knowledge, there is no ability for MS Office to open ODF files; the compatibility pack is strictly to allow older versions of Office to open the new 2007 formats.