Unlike DirectX, OpenGL is a standard implemented in driver, so OpenGL comes with GPU driver. But it really does not matter if DirectX or OpenGL is used, that something you should not need to think about as you use 3rd party engine, its Epic job to both renderes render the same thing. UE4 is made to be very portable, it even got its own standard API layer so you dont need to think on what OS you target in, only situation you need to care if you want to work on mobile, you need to lower down the graphics.