There isn't a difference, other than in terms of who uses these expressions. The normal expression, used by native English speakers and most other people, is 'spoken English'. The English which we speak is 'spoken English', just as the English which we write is 'written English'. 'Oral English' is a strange expression used only in Chinese contexts. It sounds odd to us - 'oral' is to do with the mouth, not language. I have no idea why the Chinese use this term. Take a look at this discussion: http://www.italki.com/discussion/101730

spoken English = oral English