Oatly's primary sugar, maltose, has 2.3 times the blood sugar impact as lactose, the primary sugar in cow's milk. Maltose has a glycemic index of 105, compared to 46 for lactose.[1]
Ok, but if Oatly is 100% maltose, it has only a slightly higher glycemic index than cow's milk. But the aforementioned planet oat is still lower(assuming it is 100% maltose)