There should be an option to use notify-osd as an alert, if installed. Currently the only way to do this is to write "notify-send 'NAMEOFALERT' 'EXPLANATION'" into the command box, which is a bit of a kludge.
Orage has libnotify support for alarms. You can select it any visual alarm by selecting "Use notification".
For me, that shows a popup window, instead of sending a notify-osd notification.
Orage can be linked with libnotify. If that does not work, the issue is in the distro. I will not add a third notify type alarm. Specially as you can use the OS command as you say.
check with orage -v that you have libnotify installed.