OnSong uses the enharmonic preference of the key to determine if it appears as sharp or flat. There are generally two reasons why notes may appear using sharps or flats in ways that don't work with your musical preference.
OnSong transposition works by determining the number of half steps to modulate the song based on its known key and the desired target key. For instance, if the song is written in the key of C, but the key of D is desired, OnSong calculates the transposition and modulates the notes up two half steps. The chords that are used for the key are specific to the target key. Since the key of D is a sharp key, notes that don't fall on the white keys of a piano become sharp instead of flat. In addition, any accidentals in the key will be sharp as well.
Problems can occur if OnSong does not have the correct key to calculate the transposition. For instance, if the song is declared to be in the key of F (a flat key) but is actually written in the key of G (a sharp key), then transposition may fail to apply the correct enharmonic preference to the result chords. This causes notes to be treated as accidentals while may not be expected.
The solution to this is to ensure that the song is written and declared in the proper key.
You can do this by opening the Song Editor and then tapping on the "info" icon in the menubar to open the Metadata Editor. You can then tap the key row to open the Key Chooser Menu. Lastly, tap on the Detect button in the upper right corner. This will review all the chords in the song and choose the best key to match the song's chords. Once finished, close the Metadata Editor menu and tap Done to save changes. While this isn't foolproof, it will likely correct the enharmonic preference issue at hand.
Players of stringed instruments such as guitar and ukulele tend to think in "sharps" while pianists and traditional instrumentalists thing in "flats". For this reason, guitarists may like to see a note as "C#" instead of "Db". The trouble with this is music theory. Major keys tend to be flat while minor keys tend to be sharp. Standard key signatures follow this rule except for keys with mostly black keys.
The solution to this issue is to turn on a feature of OnSong called "Theoretical Keys". By default, OnSong tries to stick to basic music theory and protect you from the more frightening aspects of music theory. If you turn on this feature, you'll be provided with a whole new row of keys that allow you to have more control over enharmonic preference.
To do this, open the Utilities Menu and open the Settings Screen. Tap on Menu Settings and then choose Style Preferences on the right. Tap on Chords to open options that effect the Style Preferences Menu Chords Tab. Locate the Theoretical Keys option and enable that. You can then access a third row of key options in the Style Preferences Menu Chords Tab that you can transpose the song into. If you don't want to see C# instead of Db, just choose the sharp key.
The last option is to force a specific enharmonic preference throughout OnSong. To do this, open the Utilities Menu and open the Settings Screen. Next, tap on Display Settings on the left and then Song Formatting on the right. Locate the Enharmonic section. You can then choose the enharmonic preference, choosing something other than the default. This will effect only keys where the enharmonic preference is not know. To force this enharmonic preference across the app, turn on the Force Enharmonic Preference option.