Invece che includere tutti i font Open Sans ho preparato tre css uno per

OPEN SANS - STANDARD

  font: https://fonts.google.com/specimen/Open+Sans 
        Open Sans - light 300 - regular 400 - semi-bold 600 - bold 700 - extra-bold 800 
  utilizzo su css
       @import '../fonts/OpenSans/OpenSans.css'; 
       font-family: 'Open Sans', sans-serif;

OPEN SANS - ITALIC

  font: https://fonts.google.com/specimen/Open+Sans 
        Open Sans Italic - light 300 - regular 400 - semi-bold 600 - bold 700 - extra-bold 800 
  utilizzo su css
       @import '../fonts/OpenSans/OpenSansItalic.css'; 
       font-family: 'Open Sans', sans-serif;

OPEN SANS - CONDENSED

  font: https://fonts.google.com/specimen/Open+Sans+Condensed  
        Open Sans Condensed - light 300 - light 300 Italic - bold 700  
  utilizzo su css
       @import '../fonts/OpenSans/OpenSansCondensed.css'; 
       font-family: 'Open Sans Condensed', sans-serif;