Important points on OAuth for SharePoint 2013

first and most important point is that OAuth is for apps and not for users in SharePoint
OAuth is Web Standard to manage permissions and security for modules like App on Web
OAuth is used to authenticate and authorize app and service
If you are developing an app on SharePoint online there is no need to create OAuth exclusively, it is integrated with the app by default
If development is done on SharePoint Onprem then we have two options:

1.Create a "high trust" app and deploy it, in this scenario tactically there wont be OAuth because there wont be any Authentication server(ACS) to send and get tokens. this type of method is used for creating internal Apps with in the network.

2.The other way is having an azure account and
   Set up SP2013 with Azure account
   create a certificate for SharePoint 2013
   register it in SharePoint server
the above all process is not so easy which it looks like.