The Oberheim OB-Xa is a polyphonic analogue synthesizer used by Prince, Lisa Coleman, Dr Fink and Boni Boyer. It is part of the Oberheim OB range that included the OB-X, OB-SX and the OB-8, all used by Prince.
The OB-Xa was first used on the 1999 album. The introduction to 1999 is probably played on an OB-Xa layered with an Arp Omni-2 . Prince continued using the OB-Xa on the Purple Rain, Around The World In A Day, Parade and Sign O The Times albums.
He would just put three or four tracks of Oberheim sounds together and use different voicings to make it thick and fat. The Oberheim was the main synthesizer he used in the studio back then. He used the Yamaha CP-80 electric grand too, but for synths, the Oberheim was the main axe. In the earlier years, he always liked to have a real strong lead line tone incorporated into his songs; that was a trademark of his. If you listen to "Delirious," from 1999, there's a real high Oberheim patch with a lot of modulation in there. It’s real lively – kind of a cross between an organ and a square wave.
Dr Fink and Lisa Coleman also used OB-Xas on the 1999, Purple Rain and Parade tours, and played them when the Revolution recorded with Prince. The OB-Xa was also used live by Boni Boyer on the Sign O The Times tour.