So does cow milk. 7g* of added sugar in a cup of Oatly, about 8g of various sugars in a cup of milk. Oatly add the sugar to more closely simulate the flavour of milk, but it tastes about as sweet... and both are way too sweet if you ask me.
*The nutrition profile is a bit more complicated as I think they add enzymes to break down starches in the oats, so you end up with more sugar even though it's not technically "added".
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]
Oatly (and most other oat milks) do NOT "add" any sugar! The sugar in oat milk comes from the starch naturally present in the oats, which is broken down into sugars by amylase enzymes during the production process.
(AFAIK the "added sugars" wording on US Oatly labels is due to a technicality of US labelling laws. It's doesn't say that on European labels.)
*The nutrition profile is a bit more complicated as I think they add enzymes to break down starches in the oats, so you end up with more sugar even though it's not technically "added".