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. 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.
While not copying originals, some of the features were taken to a better point. Continuous blendable multimode filter (HP-Notch(BP)-HP in 12 dB mode and 4-1 pole in 24 dB mode). 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.