Why is the letter o of Mobil Oil red in colour?

because it stands for OIL Because a mobil horse has a red tounge. The "O" is red in color because it stands for OIL There may not be any specific reason: the artist responsible says, "Back then I was using typography as paint, and doing it with tremendous amount of arrogance, ignoring what the Words were saying." (see http://www.typotheque.com/articles/old_guns/)