The "open o" is used in the International Phonetic Alphabet (IPA) as well as by many languages. The glyph the Unicode Consortium (and IPA) use is on the left and the glyph used by many African orthographies is on the right:

U+2183  ROMAN NUMERAL REVERSED ONE HUNDRED was added to the Roman numeral block. U+2184  LATIN SMALL LETTER REVERSED C was added for use as a Claudian letter. We do not recommend their use for anything other than what they were designed for. Please use U+0186 and U+0254 if you need an open o. Then find a font which has the serif where you want it.