Opera is a cross-interface browser. Unfortunately it can't read access keys clearly through a pre-defined script that uses some AJAX features. If the input type button has a defined accesskey..
<input type="submit" class="button" accesskey="x" tabindex="1" value="Post Message" />
...then Opera might not be able to read it. Access keys can be used instead of needing to handle the request of physically clicking on a submit button -- you could just press Alt + [x] to submit the form data.
But as you've pointed out, it could be a content filtering system that your work has installed.