Suggest you call National Merit Scholarship office of OU and clarify. But looking at the web site below, it clearly states
that 12/15 deadline is for additional scholarship. For that common app may be asking to write supplemental. But call them to get it confirmed than relying on my feedback.
http://www.ou.edu/content/admissions/nationalmerit/nmhow.html
12/15 deadline: OU freshman scholarship deadline (You do not have to submit your application by this date to receive OU’s National Merit Scholarship; it is necessary only if you wish to be considered for additional scholarships.)