Oat starch to sugar conversion in oat milk
Based on Fabby's suggestion in the comments on the question: oats can be malted (ref: Wikipedia: Malt], which produces the enzymes needed to convert starch into sugar.  I am still in the process of trying this out (and will probably edit this answer at a later time); adding some of the dried oat malt to freshly made oat milk should do the trick (ref: Oatly's description of their production process).