An OOXML supporter told me that OOXML is implemented in Microsoft Office 2008 for Mac.
As I had earlier read in a thread about Microsoft Office 2008 for Mac Microsoft Office 2008 for Mac
I have found other people talking about the implementation of OOXML on other platforms:
Microsoft Pushes Back Launch Date For Office 2008 For Mac Software
But I could not find specific issues reported. Like on the Mac, the bullet list is stored this way, on Windows it is stored this other way. Or differences in interpreting the colour markings.