Oracle the Lifesaver

Introduction

Welcome to my guide to Oracle the Lifesaver. My name is Streifhorn and, admittedly, I am not a very successful Dota player;I merely play for fun and to relax after a hard day of working and/or studying. Nevertheless, I feel that I have penetrated the wicked mind of Oracle and felt the need to share my insights with you through this guide, as there is no current guide to Oracle and I am not too happy with those (outdated) ones you can find here.

This my first guide, so feel free to comment and give advice on what to add or change.

Skills

Fortune's End:

Cast Range: 850
Effect Radius: 300
Max Channel Time: 2.5
Damage: 90 / 120 / 150 / 180
Min Purge Duration: 0.5
Max Purge Duration: 2.5
Mana Cost: 75 Cooldown Time: 15 / 12 / 9 / 6

An early skill point in Fortune's End can ensure first blood for your team because you can hold an enemy locked in place for up to 2.5 seconds, and this with it's cooldown of 15 seconds on level 1 probably twice during the early rune fights.
Use this ability to prevent enemies from fleeing from your physical damage carries or to keep enemy melee carries from getting last hits or even XP during the laning phase. Due to it's infinite range once you've started channeling, you can also ensure a kill on a low-health fleeing enemy.
Don't forget that it also purges debuffs from allies and buffs from enemies, which means that you can dash out a nice amount of damage with Purgatory Flames followed by an uncahnnelled Fortune's End on an enemy (as it removes the healing from Purgatory Flames from the enemy).

Fate's Edict:

Cast Range: 500 / 600 / 700 / 800
Duration: 3 / 3.5 / 4 / 4.5
Mana Cost: 50 Cooldown Time: 16 / 13 / 10 / 7

This skill is aweseome to either prevent the initial damage on your allies of Purgatory Flames (you'll have time to cast it twice on your friend without damaging them) or to prevent enemy carries to get last hits by disarming them. Also, cast it on fleeing allies when you know someone like Zeus is about to finish them with their magic.

Purgatory Flames:

Cast Range: 850
Damage: 90 / 180 / 270 / 360
Heal per Second: 11 / 22 / 33 / 44
Duration: 9
Mana cost: 80 / 85 / 90 / 95 Cooldown: 2.25

This is an awesome skill that let's you both heal and nuke. Spam it on friends to heal them excessively (best after Fate's Edict to prevent the initial damage) or on foes to nuke them down (alternately with Fortune's End to prevent the subsequent healing in case you don't succeed in finishing them).
With it's low cooldown, long cast range and immediate execution, Purgatory Flames is ideal to quickly farm creeps or to nuke down fleeing enemies.

False Promise:

Cast Time: 0.3+0.97
Cast Range: 1000
Duration: 8 / 9 / 10
Mana Cost: 100 Cooldown Time: 100 / 65 / 30

Now this is your ultimate lifesaver. In the early game, this is mostly used to save yourself with the help of a fully charged Magic Stick or to just keep your damage dealer alive long enough for the finishing blow on the enemy, but from mid game on, once you have enough mana and/or Urn of Shadows, you have enough healing to spam on your ally to safely have them survive after False Promise has worn off. Once you have your level 20 talent that makes the recipient of False Promise invisible, this is an ensured lifesaver because it makes you or your friend invisible for 10 seconds, so that a single charge of your Spirit Vessel is enough to refreshen them up to a new life, or for your carry to safely deal enough damage to kill the enemies around.
Additionally, the invisibility trait from level 20 talent even makes this ability a great tool to initiate ganks or to stay safe while you're healing your team mates and disabling enemies.

Talents

The talents I selected above are for the case of an all-goes-well match, meaning that you don't need the extra XP at level 10 because you have been able to ensure kills for your carry or to finish off enough enemies with Purgatory Flames often enough to be on top of them. Of course, when you find yourself having a hard time, get the XP boost to ensure the levelling that Oracle is dependent on. In contrast, the additional disabling time you can get at level 10 means even more kill opportunities for you and your team.

At level 15, the cast range is very welcome when you are not in desperate need of additional income. However, the money is still great to buy wards and other items Oracle can make very good use of. When you pick cast range, you don't need to buy Aether Lens anymore, so you can concenrate on getting other expensive items such as Scythe of Vyse or a quick Kaya to make your spells more potent.

The definite level 20 talent to choose is the invisibility perk for False Promise. As described above in the skills section, this is essential to further ensure survivability for you and your team.

At level 25, I like to pick the -5 second cooldown for fates edict because it let's you spam disarms on enemy right clickers or magic resistance on your team mates against magic wielders.
The other talent doesn't seem too good because 10 seconds time for False Promise is already plenty enough to stay alive to survive or deal the last killing blows.

Items

The starting items I like to grab (apart from courier and wards, which goes without saying for a full support) are Tango (for yourself and your carry), Clarity (so you can spam your spells from safe distance) and Magic Stick (so you can already collect some charges for quick health/mana regen).

Then you upgrade your magic stick, buy Windlace and Boots for speed, and - most importantly - get the Urn of Shadows. Depending on how well it goes during the laning phase, get the Boots last here and use Windlace for movement speed. The most important item here is the Urn of Shadows; you are a healer after all.

When you've got Urn and Boots, get the Arcane Boots and Spirit Vessel as quickly as possible. With the Arcanes you can bolster your and your team mates' mana pool, and then Spirit Vessel to improve your healing and overall survivability. Don't forget to use Spirit Vessel aggressively on enemies during team fight or to finish off a fleeing enemy. The damage output of Spirit Vessel is considerably high.
Eul's Scepter is a nice item for everyone: It gives more stats, mana regen, movement speed and a strong disable to disrupt channelling abilties of enemies or town portals (which is important for Oracle especially because Fortune's End is NOT a stun).
With those items you are practically done really needing items, apart from wards of course. So why not start with saving up for Guardian Greaves? Do so by purchasing Mekanasm (Buckler first!).

Everything else depends on the situation, whereas Scythe is always a nice item for spell casters because of the stats and the additional disable.

Playstyle

Admittedly, Oracle's skill set isn't very easy to master. However, with profound knowledge of your skills and those of all the other heroes on the map (especially your enemies'), Oracle's team will always have the advantage because you can use every skill either as an advantage for your team or a disadvantage for the enemies'.

During laning phase, Oracle is everything about helping your carry to become the killing machine your team needs as early as possible, which also means to keep the enemies low in level.
Spam Fortune's End and Fate's Edict on enemies to keep them from last hitting and finish creeps with Purgatory Flames your carry can't last hit before the enemy can deny them.
At best, keep roaming to ensure vision of the map with wards (also to anticipate ganks and deny them by casting Fortune's End in order to be able to get to safety early enough) and alternately help the other lanes to quickly get an easy kill from time to time. This sounds like a typical "easier said than done" but when you have vision of the map and can channel Fortune's End to the max (which means locking the enemy in place for 2.5 seconds at skill level 1), it actually is that easy.

During team fights, use your knowledge of the enemies' skills to cast Fate's Edict on either friend or foe; keep enemies at distance or within reach of your allies with Fortune's End; heal ranged magic wielders with your Fate's Edict/Purgatory Flames combo (because they won't care about the disarm); finish off fleeing enemies with Purgatory Flames or prevent them from fleeing with Fortune's End; and, of course, make your allies invincible by using False Promise.
This, in addition to your items (and don't forget to use Spirit Vessel offensively as well), Oracle is an indispensable team fighter and lifesaver.