Best OS for cold storage
Use TAILS Linux. From the website:


  Tails is a live operating system that you can start on almost any computer from a USB stick or a DVD.
  
  It aims at preserving your privacy and anonymity, and helps you to:
  
  
  use the Internet anonymously and circumvent censorship;
  all connections to the Internet are forced to go through the Tor network;
  leave no trace on the computer you are using unless you ask it explicitly;
  use state-of-the-art cryptographic tools to encrypt your files, emails and instant messaging.
  


Even Edward Snowden uses it.