Disable Ctrl+O in IE9 using JavaScript

Hi!

I try to disable few default hotkeys in IE9, and add new functionality.

I use this JavaScript code:

document.onkeydown = function (e) {
  if (e.ctrlKey && e.which == 79) { // disable Ctrl+O
        e.preventDefault();
    }
}

This solution works for Ctrl+N, Ctrl+W, Ctrl+B, etc., but not workin for Ctrl + O.
Can someone explain me, why? How can I prevent the default behavior?

Please help me!