Open YouTube's saved offline video (.exo file) My question is specific to YouTube app's new offline functionality. 

I have saved some video from YouTube app on my Android phone. I restored my phone OS due to some problem. But I have files that were downloaded by YouTube app stored on SD Card. Now YouTube is not showing that videos in offline list. 

Is there any way by which I can open those files in through YouTube app?

Append

I am just wondering whether youtube uses some config files to store list of downloaded videos. If so can we modify such config files and add those videos to the offline list.
No there is no way to open the .exo file since it is encrypted and the video file is split into chunks of .exo files. Even if you have one offline video stored in your phone you can find more than one .exo file under Internal Storage/Android/data/com.youtube.com directory. They are nothing but the encrypted exo files that can only be opened through the Youtube App. From the xda-developers forum post,


  1- Google is using EXO file format to save videos. This is apparently
  a file format used for system files and while we managed to copy these
  EXO files onto a computer, it can be apparently played only through
  the YouTube Android app.
  
  2- When you request a download for an offline video in the YouTube
  app, the video is downloaded in chunks. For example, a 3-minute file
  we downloaded was saved in five parts. It is possible that only the
  YouTube app for Android can make sense of these chunks and join them
  together.
  
  3- We also feel there is real-time encryption going on here. When the
  video is downloaded, not only it is broken into parts but is also
  converted to EXO format, compressed and encrypted in real time. This
  has several benefits, the one obvious benefit is that people can't
  copy-paste the video and share it indiscriminately. The compression,
  meanwhile, saves space on the disk. For example, the 3-minute long
  video that we downloaded actually consumed around 40MB data. But when
  stored on the phone, the video took only around 25mb space.
  
  4- When a user plays the video, it is decrypted in real time. It is
  possible that only YouTube's app can do this decryption.
  
  5- If you share the offline video using Bluetooth, instead of the
  actual file, a web link to the video is sent to the recipient.


However you may want to look into this Exoplayer Library on Github and see if that helps.


  ExoPlayer is an application level media player for Android. It
  provides an alternative to Android’s MediaPlayer API for playing audio
  and video both locally and over the Internet.