Found under: Settings > Song Actions > Delete
By default it is not easy to delete songs from OpenSongApp. This is an intentional feature as I don't want users blaming me for deleting their song library.
You do, however, have a few options to delete songs in the app:
Open the song you wish to delete and from the settings menu described above, navigate to Song Actions (or long press a song in the song menu and select 'Actions') and then click on the 'Delete' button. This will prompt you to confirm that you want to delete the current song. You will also be warned that you can't undo this and should have a backup of your songs just in case.
From the storage overview page, select a subfolder and delete it. This will permanently delete all songs in this folder. Again, please be careful - there is no going back if you don't have a backup! If you want to delete multiple songs, this is the quickest option. You can also create a temporary sub folder called 'todelete', then using the storage overview page, move all of the songs to be deleted into this folder. Then delete the folder.