Security-focused operating system

This is a list of operating systems specifically focused on security. General-purpose operating systems may be secure in practice, without being specifically "security-focused."
Similar concepts include security-evaluated operating systems that have achieved certification from an auditing organization, and trusted operating systems that provide sufficient support for multilevel security and evidence of correctness to meet a particular set of requirements.
Object-capability systems.
These operating systems are all engineered around the object-capabilities security paradigm, where instead of having the system deciding if an access request should be granted the bundling of authority and designation makes it impossible to request anything not legitimate.
Windows Server.
Starting with Windows Server 2008, Windows Server has added an installation option called "Server Core", in which the traditional graphical user interface is not installed. Administration, in Windows Server 2008, should rely on Windows Command Prompt. Roles and components are then installed individually. This option reduces the Windows Server footprint, the result of which is reduced demand on system resources and reduced number of components that could potentially be exploited via potential security vulnerabilities.
Later, with Windows Server 2016, Microsoft introduced a Nano Server installation option with even more reduced footprint. It was headless and did not support a locally connected keyboard and monitor. Nano Server in Windows Server 1709 (the (no longer) constantly updated sibling of Windows Server 2016) could only be installed in a container.
They have been discontinued.