Most crashes like this occur due to imported files that consume the device's limited resources. Since OnSong remembers the last song that you had opened, this can cause a crash loop.
To fix it, go into the Settings app on your iPad or iPhone's home screen. This is the same app where you set up WiFi. Next, choose OnSong under Applications on the left side. Scroll down to the Advanced section on the bottom right and turn on the Forget Last Song option. Exit the Settings app and open OnSong again and see if that helps!
You will probably want to delete the song that causes the crash to prevent this from happening again. To do that without viewing the song, locate it in the All Songs list and swipe left to right over the song, and tap the Delete button that appears.