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.