Does this Oat Milk have added sugar?
They are from the oats, which contain no sugar as a raw ingredient. Processing them to make milk converts some of the starch to sugar, so it is counted as added sugar as it does not appear in the raw ingredient.

From an article entitled Oatly challenged over “no added sugars” claim:


  ... the enzymatic process it uses to create its base oat milk (water + oats) effectively “creates sugars in situ” by breaking down oat starch into simple sugars. ... All oats in an oat beverage are processed, therefore all the naturally occurring sugars in the final product are considered added.


The same article provides an update about this from the FDA specific to the Nutrition Facts Label:


  In our Nutrition Facts Label Q&amp;A guidance issued in November 2017, we addressed sugars created through processes such as hydrolysis: When an ingredient containing mono- and disaccharides that are created through controlled hydrolosys... is added to a food during processing, those mono- and disaccharides contributed by the ingredient need to be declared as added sugars on the label.