One of the most important item in game that people usually never to pick it up on solo game. Oracle is important to gain map control as you will deny a lot of vision from enemy team, especially after you destroy outer towers and gaining control over baron area. In addition, you will be able to see enemy on stealth such as twitch, shaco, etc. It is quite expensive since it cost almost the same as doran’s item.
Who should pick up the oracle? For early game it is best for jungler to pick up Oracle’s Elixir once the complete their gp10 item, because jungler is roaming and need to remove enemy wards before the can have a good chance to gank enemy lane. On mid game if by chance jungler lost Oracle, support is the best person to pick up second Oracle so jungler and other players can rush to complete major items. Once it reach late game, anyone who completed their build and bought other elixir can pick up oracle with spare gold.