As you know, the character 'を' is primarily or exclusively used as a postpositional particle to mark the object as in '本を読む,' '字を書く,' while 'お' is widely used as a prefix to a noun in honorific or polite expressions like 'お元気でいらっしゃいますか,' 'お越しいただく,' 'お神籤,' 'お茶' and 'お神酒,' as well as a character to indicate an ‘o’ sound such as in 'おかしい(可笑しい),' 'おとす(落とす),' 'おこす(起こす), 'おんな' and 'おとこ.'
I don't see the character 'を' being used for other than postpositional use.
I think the characters 'ゑ' and 'ゐ' have become completely obsolete today in the process of natural selection, because we don't need two different characters to describe the same sound. The character 'を' could be on the same route in the long term.
I hear 'を' pronounced "wo" sometimes, actually by a female weather forecaster on NHK-TV. But to most Japanese 'を' and 'お' sound the same.
By the way, 'を' is a deformation of the Kanji '遠' and 'お' is that of '御,' both of which start with an 'o' sound.