Hi,
Here's my instinctive reaction.
'Oral' is complex, because we often begin to speak befiore we know what we are gong to say. In addition, we can't really go back and change the words we have already said.
'Written' is complex, in that we have an opportunity to think about and to create longer and more carefully constructed sentences, and to form them into intricate paragraphs.
Best wishes, Clive