Regarding the MindManager Viewer url mentioned: no, it looks like it is a regular "viewer" application, much like the free Ultra Recall viewer. Both are configured as standalone executables (not an active x control to be hosted in Internet Explorer).
Regarding OO files, I think there is an active X control that can be installed that can be hosted in Internet Explorer (and probably also in Ultra Recall), but I believe it only provides read-only access (no modification capabilities) so it would be of very limited usefulness... Here is a
link that implies the capability (as well as at least one user reporting that it doesn't always work in IE).