Newer versions of iOS allow you to configure app permissions within the Settings app after selected the application. The following items are configurable under iOS 10 or greater:
OnSong automatically links chord charts to backing tracks contained in your iTunes Media Library. Under iOS 9.3 and higher, you may be prompted for access to your library. If you decline, you must enabled media library access here.
OnSong very rarely sends push notifications, but you can customize how these notifications will be viewed under this option. You can also disable notifications from OnSong altogether.
Background App Refresh
While not currently in use, OnSong could update information un the app upon receiving a notification. You can disable this function here.
OnSong is designed to operate completely within WiFi or access to the Internet. If you are using OnSong on a cellular-enabled device, you may wish to prevent the use of cellular data on the application level. You can disable cellular data use for OnSong here.