In the beginning when Oenomaeus is shown fighting in the Pits as a young Boy, one of the patrons describes him after his victory as being, " a demon; belched from the underworld." This is the same way that Batiatus described him in the Blood & Sand episode, "Sacramentum Gladiatorum."
The reason behind Oenomaeus' inverted brand, is never explained. It is possible (stemming from his characteristics) that Oenomaeus would have preferred to have a legible view of his own brand as, self-motivation. (Seeing as Oenomaeus grew to be immensely proud of his service to the house of Batiatus)