Yup, you guessed right. We use W.O. with the same meaning as in English, especially in soccer matches. When the other team fails to appear, or there isn't enough players in one team, the other team is considered the winner by W.O. (or vencedor por W.O.).
And although most Brazilians know what is a W.O., they have no idea that it means Walk Over.
- Quem venceu o jogo ontem?
- O São Bento venceu por W.O. do Juventus.