ó 13
It is - obviously - an 'o' letter with a sharp accent. Although no longer recommended by the RAE, it has a very specific use as an adverse conjunction between numbers, to avoid being confused with a zero ( 0 ). It is useful when writing by hand, with "paper and pencil", and not so much in printing where typography can show an obvious difference. In languages where graphic ticks do not exist, a zero-crossed-one-line design is used, which is similar to the O with bar 'A' , or the Tibetan mean zero '3891;', but of course they are not the same.