LineageOS is a custom OS which you can flash onto your supported Android devices using a custom recovery or using ADB sideload. It is the successor to CyanogenMOD, which many people know about since it was around for so long. Custom OS's such as CyanogenMOD and LineageOS are created for the sole purpose of having a stock Android feel, but with added features on top of the OS which are not included with the vanilla version of Android that Google ships on their Nexus and Pixel devices.
LineageOS and many other custom OS's can be used to jump to versions of Android which haven't been released for the phone that you are on yet. This is the case for OnePlus 5 users as an alpha release of LineageOS 15.1 allows users of the device to flash the custom OS and have the benefits of Android Oreo 8.1. A downside to this method is that because these aren't official releases from the maker of the phone, somethings may not work or can be buggy.
Regardless, an alpha of LineageOS 15.1 was released by XDA Senior Recognized Developer codeworkx for the OnePlus 5, allowing for users to update to Android 8.1 Oreo. The build is unstable and not recommended for your main device. There also seems to be no clear upgrade path from the 14.1 official release to the 15.1 alpha build released. If you are determined to upgrade though, a data wipe and a clean flash are enough for a successful upgrade from 14.1 to the latest build.
codeworkx has dropped hints that he may release a single package which would work for both the OnePlus 5 and 5T once the builds become stable. As of currently not all of LineageOS's applications and features have been made available on the alpha build.
A cautionary note: If you have a PIN or password on your device, it can make it difficult for a custom recovery to decrypt your phone's data. It is recommended you read the flashing guide carefully before risking your device.