フェニョさん、こんばんは。 Its pronunciation was "wo", but "w" has been lost, as the time went by. Other than object marker, 「を」 is not used in modern Japanese. It shares the same phoneme with「お」. Since you write it in katakana, not hiragana, I assume you mean ‘loan words’, like 「ウォール街」. Council for National Language from Ministry of Education, Science and Clture showed the guideline for how to write loanwords. The below is in Japanese, but it has the table on each foreign sound should be written in katakana. http://ja.wikipedia.org/wiki/%E5%A4%96%E6%9D%A5%E8%AA%9E#.E5.A4.96.E6.9D.A5.E8.AA.9E.E3.81.AE.E8.A1.A8.E8.A8.98

わかりません。そういうものだと思うしかないのでは？