OAuth authentication follows the Social Login specification. So if a product is available with social login it will be available for OAuth as well.
Assignment of a specific product works as always.There must be a single product for that specific registration mode or a single free product in order to get it assigned to the user. Instead, if you have more then one product available for, in this case, social login user will have the usual screen to choose the prefered prodcut.
About login process from an external resource: Yes, technically is possible but you have to develop something that does everything that is done by the welcome portal. We already provide API to be able to create users, assign product to them etc... but all that manage this has to be developed by you in your APP.
In the Admin section of the HSNM, under the documentation menu, you can find examples about how to use API via VB.NET. VB.NET can be a language to use to develop your APP.