DNN added support for an OAuth in DNN8. By default this only works in HTTPS scenarios, and does not support "localhost" -to enable support for either or both of these, please add the following in the configuration node of web.config

<dotNetOpenAuth>
<oauth2>
<authorizationServer />
</oauth2>
<!-- Relaxing SSL requirements is useful for simple samples, but NOT a good idea in production. -->
<messaging relaxSslRequirements="true">
<untrustedWebRequest>
<whitelistHosts>
<!-- since this is a sample, and will often be used with localhost -->
<add name="localhost" />
<!-- Uncomment to enable communication with localhost (should generally not activate in production!) -->
<!--<add name="localhost" />-->
</whitelistHosts>
</untrustedWebRequest>
</messaging>
</dotNetOpenAuth>