How to sing O Come, All Ye Faithful in your key
O Come, All Ye Faithful spans 12 semitones (D4 to D5 in its usual key), 61 notes. That's an octave or less, so it fits most voices once it's in the right key.
Words
O come, all ye faithful,
Joyful and triumphant,
O come ye, O come ye to Bethlehem;
Come and behold Him,
Born the King of angels;
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.
Finding your key
A song is in your key when its highest note sits at or just below the top of your comfortable range. Measure your range, then move the melody so its top note (D5 here) lands one or two semitones under your easy top. Singsteady does this for you and scores every note as you sing.
18th century (John Francis Wade); English words 1841 (Frederick Oakeley). Melody and words are in the public domain. Notes transcribed from the score in Wikipedia, “O Come, All Ye Faithful” (score, top voice).