WebApp Authentication on mobile Device

Hi i am currently devoloping an app for multiple plattforms. The idea is to load a website (ASP.NET) into a webview so that the UI looks the same on multiple OS. Now my question is what is the best way to authenticate users so that they dont have to login every time they open the app. My intention now is to use OAuth but i am not sure if this fits my situation because i am not securing a WebApi. If i decide to use OAuth i will have to develop my own OAuth server. Note the app is able to use native functions

Any suggestions ?