NeXTSTEP was a major ancestor of Mac OS X after Apple acquired NeXT.
Developed by NeXT, the company founded by Steve Jobs after leaving Apple, NeXTSTEP was a Unix-derived operating system built on the Mach kernel and BSD. It combined a graphical desktop with object-oriented programming tools, the Objective-C language, Display PostScript, and reusable application frameworks—technology that strongly influenced Apple’s later software platforms.
Apple announced its acquisition of NeXT in December 1996 and completed it on February 4, 1997. Apple used NeXTSTEP and related OpenStep technology as the foundation for its replacement for the aging Classic Mac OS. The intermediate Rhapsody project combined that technology with selected Macintosh technologies before evolving into Mac OS X.
The connection was more than a codebase transfer: NeXT’s development frameworks became the basis of Cocoa, while the underlying technology contributed to Darwin and the architecture of modern macOS. The same lineage later influenced iPhone OS, iOS, iPadOS, watchOS, and tvOS. Solaris and BeOS were considered in other contexts, but neither became Mac OS X’s foundation.