Added check for Opera, loading on DOMContentLoaded instead of load. Event only added if Opera has no native support. Replacing script child instead of insert/remove.

async_defer_script.js

// ==UserScript==
// @name           defer/async for Opera
// @namespace      http://d.hatena.ne.jp/edvakf/
// @license        Public Domain
// Updated by Miles Elam <miles@geekspeak.org>
// ==/UserScript==

(function(window, document, opera) {
  
  function load_script(script, attr) {
    return function() {
      var script2 = script.cloneNode(true);
      script2.removeAttribute(attr); 
      script.parentNode.replaceChild(script2, script);
    };
  }
  
  if (!('async' in document.createElement('script'))) {
  
    // support async and defer
    opera && opera.addEventListener('BeforeExternalScript', function(e) {
      var script = e.element;
      
      if (script.hasAttribute('async')) {
        setTimeout(load_script(script, 'async'), 10);
      } else if (script.hasAttribute('defer') && document.readyState != 'complete') {
        window.addEventListener('DOMContentLoaded', load_script(script, 'defer'), false);
      } else {
        return;
      }
      return e.preventDefault();
    }, false);
  }
})(this, this.document, this.opera);