It may have been a long time coming but all that is irrelevant now. If you have been waiting on the OnePlus homegrown OS, OxygenOS, then the wait is over. OnePlus have now released the download which can be used to install OxygenOS on your device. This is a manual download only, meaning there is no OTA which will be rolling out. Also, due to the nature of this type of download you do need to have an unlocked bootloader and a custom recovery installed to be able to "flash" the update. OnePlus have stated they are working on an easier way to install the update. However, until then, you will need to go through the motions. To try and make the process is a little clearer for those new to this sort of thing, here are those instructions to manually installing OxygenOS on your OnePlus One.
NOTE: This process effectively replaces Cyanogen OS on your One. This means it will wipe your system. In doing so, it will also wipe your data, files, media and so on. It is essential you make a backup of all of your data to protect it. Not to mention, in case anything goes wrong during the process. Which it can. Although, installing OxygenOS does not void your warranty, it will if you have installed OxygenOS incorrectly. As such, proceed at your own choice and be aware that issues can happen.
Depending on the current state of your One will determine where you need to proceed from. If you are already rooted and have the likes of TWRP installed then skip to step 3. If you have yet to root, unlock your bootloader or install TWRP then start from the beginning.
OxygenOS - Click Here
TWRP - Click Here
ADB - Click Here
Step 1: ADB
You will need to install ADB and Fastboot to be able to unlock your bootloader. The ADB files can be downloaded by clicking here. The link will take you to the overall Android SDK which you will need to download. Once downloaded you will find the ADB files under "Platform Tools". Once found, extract the ADB files to your desktop and rename the file. Something like "adb", "android' or "one" will be fine. Any name will do as long as you can easily find the folder. You will also need to download the appropriate drivers for your desktop, to do so click here. The driver link should detect your operating system and only show you the appropriate files to download.
At this point, turn off your OnePlus One and boot into fastboot mode (hold down the power button and volume UP together). Connect your device to your computer via your USB cable once you see the device has booted into fastboot.
On the computer, navigate to the ADB file that you renamed ("adb", "android" or "one"). Once inside this folder, right click and hit "Open Command Prompt Here".
To unlock your bootloader (if already unlocked, then skip to Step 2), in the command window, enter the command below. At which point your device should automatically reboot.
fastboot oem unlock
By now, you should have already downloaded TWRP to your computer. Place the TWRP file into the adb renamed folder. As your device has automatically rebooted, you will again need to reboot into fastboot mode, by using the same method above. Once in fastboot mode again, to install TWRP on your system, enter the command shown below.
fastboot flash recovery <recovery.img>
The "recovery.img" should be replaced with whatever your TWRP image file is named.
Step 2: TWRP
By now, TWRP should be installed. Turn your device off by pressing the power button and reboot as normally. Alternatively, if you are still in fastboot and with an open command window you can use the command prompt fastboot reboot.
Once the device has rebooted fully, make sure a copy of OxygenOS is installed on your device. You can download this again by clicking here. This needs to be installed on your device locally and somewhere it will be easy to find. Once installed. Turn off your device again and to reboot into your newly installed TWRP, hold the power button and volume DOWN key together. It may take a little longer than usual but on this occasion you should see the TWRP boot screen launch.
Step 3: OxygenOS
Once the screen launches, you will see the following options shown in the image below.
Firstly, you will need to wipe your device. Remember this might wipe everything. Click on "Wipe" followed by "Advanced Wipe". From here click on "System" and also "Data". Now click on "Swipe to confirm wipe".
Once this is finished you will see a Wipe Successful message appear.
Now, press back until you reach the main menu and this time click on "Install".
Navigate to the OxygenOS file that you already saved to your local memory. Once you reach the zip file, TWRP will automatically recognize this is a zip and ask if you want to install. Once again click on "Swipe to confirm".
On this occasion, the process will take much longer than usual, as OxygenOS will be being written onto your device. Once it is finished you will see a 'Successful' message appear.
It is now advisable to click the 'Wipe cache and dalvik" tab which appears at the bottom left of the screen. This will clear the device of any residue files that may have been left over from your previous ROM/OS
Once this has finished. Click "Reboot System".
OxygenOS has now been installed.
It is worth noting that the first boot will take some time and as long as there were no errors or issues during the installation process. You will see it go through three important stages. The first will be the "Powered by Android" screen appearing. The second stage will be the new OxygenOS boot animation loading. Once both of these stages pass you will see the third stage "Android is upgrading". By which point everything should be fine and you will see the first "Welcome to android" page appear shortly afterwards. You are now running OxygenOS.