How to authenticate with OAuth to access EWS APIs

My web service is currently doing basic username/password authentication in order to subscribe the exchange user for receiving the events (like new mail event etc) like below:

var service = new ExchangeService(exchangeVersion)
                                  {
                                      KeepAlive = true,
                                      Url = new Uri("some autodiscovery url"),
                                      Credentials = new NetworkCredential(username, password)
                                  };

var subscription = service.SubscribeToPushNotifications(
                                    new[] { inboxFolderFoldeID },
                                    new Uri("some post back url"),
                                    15,
                                    null,
                                    EventType.NewMail,
                                    EventType.Created,
                                    EventType.Deleted,
                                    EventType.Modified,
                                    EventType.Moved,
                                    EventType.Copied);

Now, I am supposed to replace the authentication mechanism to use OAuth protocol. I saw some examples but all of them seem to be talking about authenticating the client (https://msdn.microsoft.com/en-us/library/office/dn903761%28v=exchg.150%29.aspx?f=255&MSPPError=-2147217396) but nowhere I was able to find an example of how to authenticate an exchange user with OAuth protocol. Any code sample will help a lot. Thanks.

Solution

In case someone is still struggling to get it to work. We need to upload a certificate manifest on azure portal for the application and then use the same certificate to authenticate the client for getting the access token. For more details please see: https://blogs.msdn.microsoft.com/exchangedev/2015/01/21/building-daemon-or-service-apps-with-office-365-mail-calendar-and-contacts-apis-oauth2-client-credential-flow/