Use an OAS for a small site that you can run with one to three engines, but you want the features offered by the Metasys User Interface like space-based navigation and alarm management. In this scenario, the OAS Site Director replaces one of the engines with either IP field controllers or MSTP field controllers on remote field bus with MSTP to IP routers.
Another reason to use an OAS is if you do not require long-term historical data storage.
To help you decide whether an OAS is right for your facility, consider its respective data storage and data access capabilities.