Nashville notation is supported in OnSong :) Please do the following: In OnSong > Utilities (gear icon) > Settings > Menu Settings > Style Preferences > Numbering System > On > Done. You can open Style Preferences (slider icon) > Under Transpose - make sure the "power" button the the right is On (blue). You can then slide the slider to the left until you get to the "N".
If you want to apply this to a book or your entire library you can do the following in the Style Preferences Menu (slider icon) > Apply Style (icon with the 3 dots in the upper right hand corner) > Songs > All Songs.
Hope this Helps!