OnePlus firstly released the OxygenOS for the One but it was never updated much from the initial release. OnePlus has now released the OxygenOS 2.1.4 update for the One and it won’t come to you as an OTA, you have to flash it manually.
The update however is missing MaxxAudio and relevant apps, manual camera mode, RAW file format capability, fingerprint support ( The One doesn’t sport a fingerprint scanner, that’s why), and the screen temperature slider.
This ROM is community-sourced and also, you can’t flash it via TWRP, you’ll have to follow the instructions which can be found below because if you use TWRP, you’ll lose your IMEI and radio. Also, this ROM is based on Android 5.1.1 Lollipop, not Marshmallow.
Source: OnePlus Forums (Download and installation instructions here)