the oil minder needs to know when the oil was changed. so in the dash, settings, you can reset the indicator. I had the same problem. Bought a used truck, it had just had the oil changed, I have the receipt. 3000miles later I am towing my new FW 1200 miles from home and the change oil soon message appears.... I called the ford dealer and they say 'oops, we probably didn't reset'.. do this...

Oil changes on the 6.7 have been coming in around 7,000 miles.

see here to learn how to reset
http://www.fordservicecontent.com/Ford_Content/catalog/owner_guides/11f23og4e.pdf, page 25