Omer wanted to make sure we uploaded the OneNote 2013 COM API XML Schema so customers could get it if you are programming against the OneNote 2013 API. You can get it here: OneNoteApplication_2013_xsd. It will download as a text file so please just rename it as an .xsd and then you can view it in your XML viewer of choice.
For earlier versions you can see here: OneNote 2010 XML Schema & OneNote 2007 XML Schema. Take care and happy programming!