I have setup an openstack cloud and I am able to upload/download the files through browser (logging through "horizon" dashboard) of android device, but i want to build an android app to directly connect to and transact with the cloud. Is there any APIs? Which method can i take? Please revert back. Any solution would be helpful. Thank you in advance.
There are multiple ways to connect to the API. Every OpenStack instance has an accessible REST API which allows you to make HTTP requests. OpenStack Rest API
There are also numerous SDK's to connect to the API programmatically: See here