Can I use keystone service of OpenStack to access my web application?
I know that keystone is a service used by OpenStack in order to authenticate the user in order to use other services that OpenStack has, like Horizon.
What I have in mind is Oracle single sign on, that you can create your application to the cloud and then use that service and login to your applications.
Can I do the same with OpenStack?
No, Keystone API is designed to interact with the OpenStack components as standard, so the authentication mechanism offered by Keystone haven't been designed to work as you want. What you can do is try use those mechanism and create some wrapper to fake how Keystone should interact with a OpenStack component, so you might be able to do something which allow you to cover your needs, perhaps this is an odd idea.
Not sure what you want to achieve, but there are plenty of standards and libraries to achieve a common authentication mechanism like: a one-time password token or X.509 certificate.
