[9] A prominent part of macOS's original brand identity was the use of Roman numeral X, pronounced "ten", as well as code naming each release after species of big cats, and later, places within California.

[25] Siracusa's review of version 10.3, Panther, noted "It's strange to have gone from years of uncertainty and vaporware to a steady annual supply of major new operating system releases.

The simultaneous release of two operating systems based on the same frameworks placed tension on Apple, which cited the iPhone as forcing it to delay Mac OS X 10.5 Leopard.

[47][48] macOS Sierra added Siri, iCloud Drive, picture-in-picture support, a Night Shift mode that switches the display to warmer colors at night, and two Continuity features: Universal Clipboard, which syncs a user's clipboard across their Apple devices, and Auto Unlock, which can unlock a user's Mac with their Apple Watch.

[49][50][51] macOS 10.13 High Sierra, released in 2017, included performance improvements, Metal 2 and HEVC support, and made APFS the default file system for SSD boot drives.

Big Sur brought major changes to the user interface and was the first version to run on Apple Silicon, based on the ARM architecture.

On top of Darwin, Apple layered a number of components, including the Aqua interface and the Finder, to complete the GUI-based operating system which is macOS.

For the Mac transition to Intel processors, it was modified so that developers could build their applications as a universal binary, which provides compatibility with both the Intel-based and PowerPC-based Macintosh lines.

[94][17] A number of macOS applications continued to use Carbon for some time afterwards, especially ones with heritage dating back to the classic Mac OS and for which updates would be difficult, uneconomic or not necessary.

[117][118][119] These self-made computers allow more flexibility and customization of hardware, but at a cost of leaving the user more responsible for their own machine, such as on matter of data integrity or security.

[122] These rumors subsided until late in May 2005, when various media outlets, such as The Wall Street Journal[123] and CNET,[124] announced that Apple would unveil Marklar in the coming months.

Third party emulation software such as Mini vMac, Basilisk II and SheepShaver provided support for some early versions of Mac OS.

Rumors of Apple shifting Macs from Intel to in-house ARM processors used by iOS devices began circulating as early as 2011,[135] and ebbed and flowed throughout the 2010s.

[140] ColorSync, a technology introduced many years before, was improved and built into the core drawing engine, to provide color matching for printing and multimedia professionals.

New interface elements were integrated, including sheets (dialog boxes attached to specific windows) and drawers, which would slide out and provide options.

[160] Sync Services allows applications to access a centralized extensible database for various elements of user data, including calendar and contact items.

After the immense buzz surrounding Mac OS X 10.2, codenamed "Jaguar", Apple's product marketing began openly using the code names to promote the operating system.

Panther included as many or more new features as Jaguar had the year before, including an updated Finder, incorporating a brushed-metal interface, Fast user switching, Exposé (Window manager), FileVault, Safari, iChat AV (which added video conferencing features to iChat), improved Portable Document Format (PDF) rendering and much greater Microsoft Windows interoperability.

This operating system functioned identically on the PowerPC-based Macs and the new Intel-based machines, with the exception of the Intel release lacking support for the Classic environment.

For most users, the most noticeable changes were: the disk space that the operating system frees up after a clean install compared to Mac OS X 10.5 Leopard, a more responsive Finder rewritten in Cocoa, faster Time Machine backups, more reliable and user-friendly disk ejects, a more powerful version of the Preview application, as well as a faster Safari web browser.

It brought developments made in Apple's iOS, such as an easily navigable display of installed applications called Launchpad and a greater use of multi-touch gestures, to the Mac.

[134] Changes made to the GUI include auto-hiding scrollbars that only appear when they are used, and Mission Control which unifies Exposé, Spaces, Dashboard, and full-screen applications within a single interface.

[227] Updates included enhanced voice control, and bundled apps for music, video, and podcasts that together replace the functions of iTunes, and the ability to use an iPad as an external monitor.

[231] macOS Monterey was announced during the WWDC keynote speech on June 7, 2021, and released on October 25, 2021, introducing Universal Control (which allows input devices to be used with multiple devices simultaneously), Focus modes (which allows selectively limiting notifications and alerts depending on user-defined user/work modes), Shortcuts (a task automation framework previously only available on iOS and iPadOS expected to replace Automator), a redesigned Safari Web browser, and updates and improvements to FaceTime.

The Spotlight Search bar and all app icons were made even more rounded, smoother animations were implemented for notifications and the lock screen, and new slow-motion screensavers of different locations worldwide were added.

[237] macOS supports additional hardware-based security features on Apple silicon Macs:[238] macOS's optional Lockdown Mode enables additional protections, such as disabling just-in-time compilation for Safari's JavaScript engine, blocks FaceTime calls unless you have previously called that person or contact, location information is excluded when photos are being shared, Game Center is disabled, and accessories have to be approved and your Mac has to be unlocked.

[243][244] In an analysis, Hackintosh developer Mykola Grymalyuk noted that RSR updates can only fix userland vulunerability, and cannot patch the macOS kernel.

[245] Lockdown Mode is an optional security feature designed to provide extreme protection for users who may be at risk of targeted cyberattacks, such as journalists, activists, and public figures.

[250] Worms, as well as potential vulnerabilities, were noted in 2006, which led some industry analysts and anti-virus companies to issue warnings that Apple's Mac OS X is not immune to malware.

In larger scale advertising campaigns, Apple specifically promoted macOS as better for handling media and other home-user applications, and comparing Mac OS X (especially versions Tiger and Leopard) with the heavy criticism Microsoft received for the long-awaited Windows Vista operating system.