OB-Xd is based on the Oberheim OB-X. It attempts to recreate its sound and behavior, but as the original was very limited in some important ways a number of things were added or altered to the original design.
The OB-Xd was designed to sound as good and as rich as the original. It implements micro random detuning which is a big part of that sound. However, it was not designed as a self-contained completely independent soft-synth. It needs to be contained within a VST framework where things like transposition, automation, layering, arpeggiation, etc., are available. Also, like many synths of the OB-X's generation, the OB-Xd has no internal effects so its sounds and textures can be greatly enhanced by the use of additional processing like chorus, reverb, delay, etc.
From the manual:
"Thanks to 2DaT who created this little marvel. But we would be remiss not to thank all the members of KVR who participated in the Obxd forum thread. While the heart and soul of its design and development is all 2DaT, much discussion and debate went into it's graphic layout and the implementation of some of its features."
"So thanks to all who participated in its creation and also to the various people who stepped up to create excellent free patches for the Obxd."
Source code is available at https://github.com/reales/OB-Xd