The latest version of OnSong is now available on Mac OS devices, running OS Big Sur to newer. Like the iPad OS version of OnSong, you will need a subscription to run this version. If you already have Premium or Essentials, then you are good to go. Just sign into your account and you will have access to whatever subscription you have.
Note that, if you already have OnSong on your iPad and you download it on your computer there is no way to sync between the devices. Meaning, you will have a blank library on the Mac but it's easy to share your library and get it set up. You will want to create a backup of your Library, then import and restore it on your Mac app.