In addition to TinkerOS, ASUS has another official operating system for the Tinker Board: Android. It can be easy to forget that Android is actually an open source operating system based on Linux. As a result, anyone can build their own distribution of it, just as with desktop Linux distributions such as Debian.
At the time of writing, the version of Android that ASUS has released for the Tinker Board is based on Android version 7, known as Nougat. Although an older version, it is still supported and receives security updates, making it a safe operating system to use connected to the Internet.