OnSong itself does not provide content such as music sheets, chord charts, or lyrics. Instead it provides many tools that allow you to import (and later edit) content from various sources.
For instance, you could import content from a Dropbox folder, or from your computer using iTunes File Sharing. You could also view content in other apps such as Mail and then copy an attachment into your OnSong library. Additionally, you could browse certain websites and use the "Add To OnSong" Safari extension to flag that song to be imported the next time you open OnSong.