In the ancient world, oracles were temples where people would go to get their fortune told (yes, the word referred to the building the fortune-teller worked at, not the fortune-teller herself). Unlike modern fortune-telling, everyone involved took that shit seriously, and people would pay large sums of money to listen to a woman, probably high on various poisonous fumes, ramble cryptically at them.
Paizo's attempt to port over, buff up, and grimdark over the Favored Soul, in a nutshell. Like most full spontaneous casters, they are Tier 2, capable of breaking the game with the right (or wrong) spells, but can't trivialize everything thanks to their low number of spells known.
Sometimes, in Golarion, a god will attempt to divinely inspire a prophet to go forth and do their will among the people. This divine power gives them many powerful abilities, but the message is usually a little garbled, and every oracle is a little off in the head. It also warps them a little: some go lame, others go blind, a few go mad, and all sorts of other side effects. It's a bit of a mixed bag for the god in question. An Oracle doesn't have to worship the deity that empowered them. Indeed, the iconic (example character) Oracle is an atheist kicked out of atheist land for her powers.
The sorcerer to the cleric's wizard, oracles are divine spellcasters that cast from a pool of select spells spontaneously, rather than preparing them in advance. Like sorcerers, they lose out to their spell-preparing counterparts on pure versatility, since they can't channel energy and have access to fewer spells, but, also like sorcerers, they get more spells and access to a mystery, which is like a cleric's choice of god but much more badass. It's their equivalent to the sorcerer's bloodline powers, and gives them a ton of new class abilities known as Revelations: the mystery of metal, for instance, grants the ability to wear heavy armor and wield martial weapons on top of metal-related spells while Heavens boosts Color/Prismatic Spray. An Oracle gets 6 revelations over 19 levels, plus another for every feat spent on Extra Revelation, out of the 10 that can be picked for each mystery. Very few mysteries actually have six good, let alone great, revelations however, and it's often viable to select archetypes that trade away mysteries, so long as a proper revelation is gained early enough to gain more via feat.
They also suffer from an oracle's curse, like going blind or having lots of grumpy ghosts constantly knocking shit out of your hands. While it usually comes with significant mechanical drawbacks (even something like Tongues, which restricts the languages you can speak in combat, cuts off [language-dependent] spells and restricts what summons you can control), the curse also unlocks new spells and powers for the oracle in question (though some of the less painful ones also don't get much in return), and as they rise in level and learn to live with it it becomes less and less of a liability.
One little utilized difference from Cleric is that, unlike Clerics, Oracles aren't prohibited from casting spells opposite of their deity's alignment. Most of these spells are useless if you aren't the matching alignment, as they only work against opposite alignment or do strictly evil things like dooming a soul to Hell regardless of alignment. This means the primary use for this is playing good aligned Oracles cursed by evil deities without losing access to [Good] spells. There are a few gems you can pick up though like Infernal Healing, Screaming Flames, Hellfire Ray (if you take merciful spell), Death Clutch and undead creation stuff for good Oracles or Angelic Aspect and Burst of Radiance for evil Oracles.
Two recurring revelations that show up in multiple revelations are of note for character op. One is one that summons weightless armor. While this armor is behind the curve when compared to magic armor, it's free and the part all the AC is armor bonus, so its eligible for Magic Vestment to add enhancement bonus and make it better than normal armor. Another, less common, recurring one is making your armor class depend on charisma instead of dexterity. Normally this isn't too exceptional, you need dexterity for initiative, but this can be changed with the Noble Scion (War) feat, which changes even that to also be based on charisma, letting you dump dexterity.
Pathfinder made the spontaneous casters viable choices, and oracles are powerful and a lot of fun to play, especially because you get to roleplay what essentially amounts to a crazy homeless person with magical powers, and maybe a heart of gold if you like. They're also less awful for dual-classing with the Sorcerer if you want to go for a Mystic Theurge, because then you can just max the fuck out of your Charisma score instead of juggling two abilities.
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|