Found under: Settings > Display > Change fonts

Many songbook apps restrict the font used in the song display to monospaced fonts.  This makes it easy for them to line up chord lines to lyric lines.  OpenSongApp, of course, allows full customisation of the fonts used in the song display.  You can select from literally hundreds of fonts that are freely available on all from the comfort of the app.

You can also add your own .ttf or .otf fonts into the OpenSong/Fonts/ folder.  These will show at the top of the font list with the prefix 'Fonts/'.  This is also important if you do not have Google Play Services installed (required for Google Fonts, Cast and Nearby features in the app).

Fonts that can be changed are:

When you change a font, the preview will update to show you what to expect. 

You can also browse the available fonts by clicking on the 'web' icon next to the font name.  This opens up all available fonts beginning with the letter 'A' by default.  You can choose other fonts, by clicking on the alphabetical index on the left of the screen.

Clicking on a font from the web preview will set this font.

Be aware that some fonts include extra line height (look at the output in this screenshot for Gaegu and Galada fonts compared to the Gafata font).  This will appear in the app as well.  You can, however, try to trim this extra line spacing using the option in the Display > Song Display menu