LineageOS is a free and open-source operating system for smartphones, tablet computers, and set-top boxes, based on the Android mobile platform. LineageOS is a popular free and open source operating system. It was forked from CyanogenMod in 2016. A free and open-source operating system for various devices, based on the Android mobile platform. If you’ve been considering giving your phone new life with a custom ROM, LineageOS is one of the most popular ones available today.