The OpenMRS server is a standard java web server. To access OpenMRS as a client, you simply need a web browser that can read web pages. The web browser can be on a computer or mobile* device and can be using any operating system you desire.
Clients are usually just entering data via forms. The submitted forms are turned into encounters. To fill out a form you need to have one of the form entry solutions. There are different requirements for each:
*Please note that all features of the OpenMRS server/website might not be available on a mobile browser. See the Mobile page for different ways of interacting from a phone or mobile device.