The first official public version of Mac OS X was launched by Apple on March 24, 2001. It wasn’t a success at first, but it powered the Mac through the next 19 years until macOS Big Sur.
From Pink and Taligent, through Copland and Gershwin, Apple had repeatedly tried to make a new operating system to keep the Mac up to date. Typically it had multiple versions in the works, some intended to address short-term issues, others to ready the Mac for the future.
When none of these years-long projects led to a shipping product, Apple went shopping for an OS, and ended up buying Steve Jobs back into the company.
Acquiring NeXT and therefore its NeXTSTEP operating system, was what paved the final road to OS X. On the way, it was briefly expected instead to lead to Rhapsody, the codename for an OS that would run classic Mac apps within NeXTSTEP.
Read more at Appleinsider.