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The answer is you can use either. They both have their own advantages.

OpenStudio is an Object-Oriented Application Program Interface (API) for EnergyPlus, and also has a Graphical User Interface (GUI). It incorporates user scripts in the Building Component Library which can make some processes much easier to do, though there are still EnergyPlus objects, particularly HVAC systems, that are present in EnergyPlus but not yet OpenStudio. Reference @MDahlhausen's answer here: https://unmethours.com/question/22108/openstudio-vs-design-builder/

You are correct that OpenStudio uses EnergyPlus. But you can also use EnergyPlus directly. It may be a bit less user-friendly for most people, but it does include access to every object. It does not have interaction with the Building Component Library.

I would recommend using OpenStudio if you are a new user. Then if there are features that you find you would like to use that are not available in OpenStudio, you can either write a script to use them in OS or move over to EnergyPlus.