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. 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.
MIDI keyboard required for standalone app. GarageBand is recommended to setup MIDI Bluetooth keyboard via Advanced Settings.