Not just OpenStep, but Rhapsody! It was Apple's original transition plan for Mac OS X, basically OpenStep/Mach 5.x with a Mac OS 8-style Platinum UI done in NeXT-style Display PostScript complete with draggable menus. It's my favorite Apple product of all time just for the "what could have been" factor, but the Adobes of the world balked at "rewrite your apps in Yellow Box" so we got Carbon and friends in the rebooted Mac OS X project. There was one retail release as "Mac OS X Server 1.0", but the "Premier" desktop-user-focused version was canceled. I like to run Rhapsody on my Power Mac G3 Blue&White because that particular machine shipped with Rhapsody (in a Server G3 configuration) and is extremely well supported: https://cooltrainer.org/images/original/michiru-rhapsody-201...
And here are some good starting points for more info:
