Is it permissible to benefit from a pictorial dictionary by converting it into a smartphone application with added audio and mentioning its original source (Oxford) and the developer's name, then selling it?

If what you intend to do—mentioning the source and adding new changes and applications—does not infringe upon the right of the dictionary’s source, then there is no blame upon you for it. It is preferable to consult specialists to inquire about what conflicts with the intellectual and moral rights of the author.