Oracle is a ranged intelligence spellcaster that is typically played as a support hero.
He has tremendous burst nuke and healing potential, and an incredibly powerful single target ultimate, but has a very steep initial learning curve, so is not recommended to newer players.
Skills
You want a point in each of your abilities at level 3.
Fortune's End is skilled and maxed second.
Fate's Edict is skilled at level 1 but maxed last.
Purifying Flames is skilled last but maxed straight away.
False Promise is always skilled (levels 6, 11 and 16).
Items
Starting Items: Observer Wards and/or Courier, Tangos, Healing Salve, Clarities or Iron Branches.
Early Game: Flying Courier, Observer Wards, Boots of Speed, Magic Wand, Urn of Shadows.
Core Items: Arcane Boots, Eul's Scepter of Divinity.
Luxury Items: Force Staff, Rod of Atos, Drums of Endurance, Scythe of Vyse, Orchid Malevolence, Heaven's Halberd, Ghost Scepter.
Situational Items: Sentry Wards, Dust, Gem, Smoke, Mekanism, Pipe of Insight, Necronomicon, Vladmir's Offering, Diffusal Blade, Linken's Sphere.