As of ActivEdit 4.0 Opera is not officially supported as a browser for using ActivEdit, but Opera will run the ActivEdit applet, when java is installed. Our interal tests show the applet working on Opera 7.23 on Windows XP.
Because Opera contains MSIE compatable in its user agent ActivEdit expects the browser to render the ActiveX editing control, Opera however cannot. Using the applet="always" attribute should allow Opera to display the Java Applet (but it will also force IE to display the applet as well).
Another workaround is to edit the browser detection such that if the user agent contains Opera to not treat it like MSIE. This requires an open source license.
Expect to see support for Opera in future versions of ActivEdit.