I don't know of any Office-wide keyboard shortcuts for changing the font and looking at the list of keyboard shortcuts in OneNote 2010 there actually doesn't seem to be one.
Assuming you are using Office 2007 or 2010, you can, however, add the font dropdown to the quick access toolbar from the context menu you get by right-clicking on the dropdown's arrow. Once in the quick access toolbar, you can get to it by pressing Alt and the number for its position in the toolbar (e.g.Alt+5 if it's the fifth element, though the number also appears next to it, when pressing Alt). This will set focus to the dropdown, though you would still have to type the beginning of the font's name and press Enter. It would save you reaching for the mouse, however, and probably be less of an interruption to your writing flow. This, by the way, does not only work in OneNote, but also the other Office applications.
If you were using Word, an even better way to accomplish this would be to first create a style for the kind of formatting you want to apply (font size, bold/italic/underlying etc.). This is always a good idea instead of applying the formatting by hand, because it makes sure passages are formatted consistently and it allows you to reformat everything at once. Then you could assign that style a custom shortcut key like so.