The Oberheim OB-SX is a polyphonic analogue synthesiser used by Prince and Lisa Coleman. It is part of the Oberheim OB range that included the OB-X,OB-Xa and the OB-8, all used by Prince.
It is similar to other keyboards in the OB series, but had a selection of presets rather than having a fully programmable interface. Prince liked working with these presets.
As well as using one in the studio had his own OB-SX on stage on the Controversy tour.
Prince was incredibly bold in the way he would just use a preset and then brighten the fuck out of it! He would turn the filter way up so the sound would cut through the mix. I still have the OB-SX from those days. It’s one of the most simple Oberheims - there are just a few knobs on it, but that was the most quintessential keyboard from that era.