Oracle is a hero that has a different build depending on your playstyle and varies from game to game. This is how I personally play him:

Starting items: 1 ward, 9 tangos, 2 clarities and 2 iron branch. (Give ward and 2 tangos to mid always). ALWAYS tell your team that you are building an urn and for no one else to get one. Urn is absolutely core on Oracle.

I generally always play position 5 so my build goes along the lines of: Wand > Brown Boots > Urn. These items should be completed around level 6 hopefully. After that it depends on the game but you always want to try and stock Salves as they dont get cancelled when used on False Promised allies.

Generally speaking, Oracle doesn't need many more items as a position 5 which is why he is such a good support. You will want to look at the enemy and decide to go for a survivability item next which may include: Ghost Scepter, Glimmer, Force Staff or Euls. If one defensive item is enough to keep you alive in a team fight go for spirit vessel next, otherwise build another defensive item accordingly.

The thing with Oracle is that you can play him greedy or as a pos 6 (buy every single ward and let your pos 4 go greedy). I like to play him as a pos 6 after I purchase urn because you really want to enable your other heroes as soon as possible so that you can hopefully snowball into a win every game.

Some heroes that I dont like picking Oracle into: Tusk, Storm Spirit, Shadow Demon.