By default, OnSong places a 3/4" margin on all songs. This provides space for navigation zones as well as white space for your eye to look more like a traditional sheet of paper.
You can adjust the margins to remove or add space around your text by going into the Style Preferences Menu by tapping on the sliders icon in the Menubar and then choose the Document tab. This will allow you to adjust the margins on your song down to zero if desired. You can then apply that style to other songs by tapping on the "..." icon in the upper right to open the Apply Style Menu. From there you can choose "All Songs" as well as "Future Songs". This will set the margin the same to all songs in your library as well as any new songs you create or import.