If you are using plain-text files for your songs that use Bracketed Chords or Chords Over Lyrics, OnSong can detect those chords and allow you to not only format those, but also display chord diagrams. This is not available for imported files such as PDF, Microsoft Word, or other word processing documents since those are displayed "as-is" and OnSong cannot modify the display of those file types.
To do this, open the Style Preferences Menu by tapping on the sliders icon in the Menubar and then choose the Chords tab at the bottom. Locate the Diagrams section and the power button on the right. Tap to turn on chord diagrams. You can also change the position of those chord diagrams in the Diagram Position section. You can choose Below, Above or Within, which replaces the chords of your song with the chord diagrams.
You can also choose the type of instrument that you want chord diagrams to be displayed in the Instrument section.
If you decide that you want all of the songs in your library to display these chord diagrams, simply tap the 3 dots in the upper right corner of this menu to open the Apply Styles Menu. Then tap on All Songs to apply this setting to other songs in your library, or tap on Future Songs if you want future songs that you import into your library to display chord diagrams.