An operating system is complex and the security is essential but could be one of the most difficult things to do because many operating systems are the target for a lot of hackers. The security in the OS could start in create a strong authentication for example with simple passwords, cards or biometric filters. But not all systems have this type of security, like user of operating systems I know that systems like Windows or Ubuntu have good security, but many factors are necessary to be secure, the systems have their own security like firewalls, anti-malware, authentications, filters, protocols, keys, etcetera, but the users are play an important role in the security because many time the phishing, social engineer are methods where the user install o put in risk the system.
Talking about Computer and Information Security 