OpenGL is not just it. In fact, there is quite a bit of them:
Above all, OpenGL is what it says only, while DirectX is DirectDraw, Direct3D, DirectSound, (DirectMusic?), and some more i don't remember.
These are all the sources are provided by "Khronos Group", the owner of the OpenGL API since July 2006.
- OpenGL, a cross-platform computer graphics API
- OpenCL, a cross-platform computation API.
- COLLADA, a file-format intended to facilitate interchange of 3D assets.
- OpenGL SC, a safety critical profile of OpenGL ES designed to meet the needs of the safety-critical market
- OpenKODE, an API for providing abstracted, portable access to operating system resources such as file systems, networks and math libraries
- OpenGL ES, a derivative of OpenGL for use on mobile and embedded systems, such as cell phones, portable gaming devices, and more
- OpenVG, an API for accelerating processing of 2D vector graphics .
- OpenMAX, a layered set of three programming interfaces of various abstraction levels, providing access to multimedia functionality
- OpenSL ES, an audio API tuned for embedded systems, standardizing access to features such as 3D positional audio and MIDI playback
- EGL, an interface between Khronos rendering APIs such as OpenGL ES or OpenVG and the underlying native platform window system 
- OpenWF, APIs for 2D graphics composition and display control
- OpenML, an API for capturing, transporting, processing, displaying, and synchronizing digital media