The Oberheim OB-Xk is a MIDI keyboard controller from the mid-eighties. Originally designed to control the Matrix and Xpander modules, it uses MIDI and works fine as a controller for any MIDI gear. It's got 61 keys that respond to velocity and aftertouch, two OB-style Pitch/Mod controllers, and one assignable continuous control slider. From the front panel you can directly access the first 100 single patches and/or 100 multi patches of your MIDI module. Any additional patches must be selected by another MIDI controller.
Other features include the ability to transpose up or down by up to 2 octaves or by half steps. The keyboard can be split into 3 zones. There are basic Hold and Chord memory functions and an Arpeggiator section with pretty decent features (it can use internal or external clock). The OB-Xk can also send a sequencer Song Select, Start and Stop commands. It's built with Oberheim's classic & vintage look: solid black metal case with wood end-cheeks. Definitely a unique controller to have around, although severely limited by today's standards.
Images from Synth Site
Thanks to Scott Marcotte for providing information.