On a charge of    
          : On an accusation of

On a large scale
          : Big amount / size / level

On all sides
          : From every direction

On edge
          : Serious or worry with the coming situation

Out of health
          : Not healthy

Out of the blue
          : Don't feel sad anymore

On ice
          : Stop or keep it for the future

Once and for all
          : Hope this time will be the last / hope to have the conclusionl

On the second thought
          : Change the mind about something

On the tip of your tongue
          : Try to remember something but not quite remember it

Off the hook
          : Out of difficult situation

One of a kind
         : Special / Only one among all

One bird in the hand is woth two in the bush
         : Better to keep small thing you have than to leave it for something            better

Out of pocket
         : Out of time / Out of contacted

Out of the box
         : Varied from the normal. Usually used with thinking