onCourse runs just fine on the latest Apple operating system, but there is one setting you'll need to change in order for onCourse (and many other applications) to run.
In the Security panel within System Preferences, you need to choose "Allow applications to be downloaded from Anywhere". This option is the same level of security as found in all previous versions of OSX. Although you will see a scary warning, there is no additional security concern when compared with running onCourse on 10.7 or below.
onCourse will shortly be paying the appropriate Apple tax to get rid of the warning message and allow you to choose the middle option. However we have no plans at this time to put onCourse on the Apple App Store itself.