OXYGEN OS (INSTALLATION STEP BY STEP) - NO ROOT - EASY

First thing,
The original post from One Plus is still required for downloads so here is the link:
OXYGEN OS INSTALLATION GUIDE

From there download the OS.
A Desktop/Laptop PC is definitely required without which this is not possible.

Download all the links given below, if you have not downloaded some of that from the above link of One Plus installation guide:

1. OXYGEN OS

2. TWRP RECOVERY IMAGE

3. ANDROID SDK
for other then windows please follow this link GETTING STARTED ANDROID SDK

4. JAVA SE DEVELOPMENT KIT

We will start with installing the Android SDK to the system and installing the required tools.
Java is required for running Android SDK.
The Android SDK once you download and start installing while installing or you can download the Java prior to that from given link no. 4 above.


Step 1.

Hope you have downloaded all the links above.
First install the Java Se development kit and install in lets not get into getting it installed in midway.

So once you have started installing android SDK you will get to this screen if after checking for JAVA during the process.

 

Make sure you change the path as given above.
This is not the default path that will be shown on your system.
Do it as this save your time in next steps.

STEP 2.

After the installation you will land up on the screen shown below

 

As you can see the items marked as Installed are required for installation which you require for Flashing the ROM.
When you will first load it they will be marked as not installed.
So select the
a. ANDROID SDK PLATFORM TOOLS
b. GOOGLE USB DRIVER

Once you click install and accept the certificate in the next pop-up window these tools and drivers will be installed.


STEP 3. The Next step is to set the path for using android sdk:

You'll need to update the system Environment Variable settings instead. Here's how it's done on a Windows 8 machine:

• Hit the Start key on your Keyboard.
• Start typing the words Environment Variables.
• As you type, you'll see the choice to Edit the system environment variables. Choose it.
• In the Environment Variables window, select the PATH line item in the Systems Variables section, then click the Edit button.

Add the full path to the Android SDK tools and Android SDK platform-tools folders in the edit box, separated by a semi-colon. It should look something like this:

C:\Android\sdk\tools;C:\Android\sdk\platform-tools

For older versions of Windows, refer to the documentation that came with your computer for assistance on setting the PATH. And, again: If you've installed your sdk somewhere other than \Android, you'll need to adjust accordingly.
Note: leave other paths as it is just add the above given values.


STEP 4.

Connect the phone to the system if you have drivers properly installed or not.

For that once you go to Device and Printer on System settings you will see a unknown device or if you have already enable USB debugging the you will see a MTP device.
(Enable Developers option by clicking build name 7 times in About tab in settings to select the Enale USB Debugging option)
If in both cases you see a exclamation mark then it means that you have improperly installed drivers.

If above steps are cumbersome please follow the link below
ADB, FASTBOOT AND DRIVERS

STEP 5.

Once you are through all this the main steps starts now.

Connect the phone to the system

Open run command(XP) else click start Menu.

Type CMD then command prompt will pop up.
Remember to be the running as administrator.
Type in command prompt

Run command

fastboot devices

If this returns you to the back same path then everything is running perfect.

fastboot oem unlock


So now before proceeding copy the recovery image downloaded in Link 2 TWRP recovery image. Paste that file into C:\Android\sdk\platform-tools and rename to twrp.img

Secondly Copy the OS in the root folder and Unzip as it will be required when you are once in the recovery mode.
You can copy in any folder in your phone but do not extract the files to root folder of android.
If you are familiar you can even use a USB OTG to do the same as TWRP supports OTG USB.

STEP 6

open a command prompt and type:

adb reboot bootloader

You should now be in fastboot mode.The phone will be will start with a logo of Andro as Fastboot.
If not previously done in earlier step copy the TWRP image file into the same folder as your adb and fastboot binaries. Rename the image to twrp.img and type:

fastboot flash recovery twrp.img

This will install TWRP recovery . Then type the next command.

fastboot reboot

When you press this command instantly hold the Volume down and power button on your phone else it will restart and custom recovery will take over the phone.
Keep on holding untill it vibrates.
Once you leave hopefully it will land into TWRP recovery.
If it lands onto the custom recovery of your phone then repeat the installation of Step 6 as the custom recovery took over the TWRP recovery.

Once this is done follow the steps on the One Plus Installation guide for completing the process.

Hope this guide will help you atleast a bit as i have not thoroughly covered all the points.
If you are unsure even after these steps please don't proceed with installation.
As it was a R&D process for me as well so i will not advice those who are not comfortable with computer installations and processes to proceed.