Dark Souls - Boss Guide - Ornstein and Smough

You were killing the bosses in Dark Souls easily enough but now you have met Ornstein & Smough and pretty much saying "what the hell is this shit?"
Don't worry we all haven been there, Ornstein and Smough (O&S) are usually the first big obstacle for Dark Souls players simply because too much is happening at the same time.
You also need to keep an eye on both bosses pretty much the whole time which takes some getting used too.

Lore
Ornstein is the captain of the Four Knights of Gwyn, and presumably, the leader of Gwyn's knights. To slay the dragons, he wields lightning power and imbues his Dragonslayer Spear with it. Despite his large size he is extremely agile, using his spear and projectiles to reach great distances with his offensive onslaught.

Smough is the royal executioner of Anor Londo. He longs to be ranked with the Four Knights, but his cruelty, including using his victim's bone as his food's seasoning, forever denied him the position.
He is far slower than Ornstein, but extremely strong, and his attacks are difficult to avoid at close range.

Look at this magnificent duo that will give you nightmares.

Drops
Kill Ornstein last:
Soul of Ornstein
Leo Ring - you can loot it from where his body was.
Kill Smough last:
Soul of Smough

Summoning
Now you can make this abit easier by summoning Solaire or a Sunbro.
You can find the summon sign of Solaire at the room with the Royal Sentinels, on the top of the first set of stairs near where the Silver Knight archer is.

Weakness / Resistance
Both knights have seperate weakness and resistances which are as follows
Ornstein:   
Weakness: Magic, Fire, Occult   
Resistance: Lightning
Smough:       
Weakness: Magic, Lightning (First Phase)
Resistance: Lightning, Fire (Second Phase)

Dangerous Attacks
Overall all their attacks are pretty dangerous, but these attack can easily 1-shot if you aren not paying attention.

O&S have their own attack and they differ alot from each other, you should learn atleast know the deadly ones like Spear Lunge (Ornstein) or Joust (Smough) to know when to dodge or simply...run away

Spear Lunge
Ornstein glides towards the player, and then stabs with his spear. The attack can be blocked or dodged, you can also counterattack.

If you don't feel comfortable with blocking or counterattack, just dodge it which is most safe option for the first time you meet this boss.
Even if you think the pillar will save you from his lunge attack just dodge when he is lunging towards you because sometimes he will go through the pillar and hit you.

Joust
Smough charges at the player with his hammer, then swings it upwards diagonally when he reaches the player. This deals heavy damage, especially when in lightning form. It can be avoided by running directly away or with the right dodging speed. Blocking will break the player's guard if they are using a shield of inadequate stability. While pillars will block Smough's movement, the hammer itself will go through the pillar.

When you see Smough doing this attack just run away and create some distance, just make sure you don't corner yourself while Ornstein is gliding around the room.

Good to know

• The player must fight both at the same time. When one is killed, phase 2 will begin and the other absorbs the power of the fallen one and receives HP, damage, and resistance boost.
• He will also be back to full health and take on a physical characteristic of his fallen comrade: Ornstein grows in size while Smough is imbued with lightning.
• You can not kill them at the same time, so just focus on one generally Ornstein is easier to kill first and Smough's second phase is a bit easier then Ornstein's.
• You will miss the Knight's Honor achievement and the Leo Ring if you kill Ornstein first though.
• Keep your lock-on on Ornstein whether you kill him first or not.
• His attacks have the potential to hit you even when he's far away and sometimes go through the pillars see above (Spear Lunge).
• Smough's attacks are predictable, easily avoided, and most importantly, easy to see even if you're not locked on because of his size and slowness.
• If you are not comfortable with their attack patterns and keep walking backwards with your shield up untill Ornstein lunges at you.
• Hit Ornstein a couple of times and just back up again; don't be greedy he will lunge at you soon enough, keep Smough away from Ornstein and yourself.
• Try to keep both bosses around the outside of the room, this way you will have both of in view most of the time.
• Since you are locked on Ornstein you really just need to see Smough from the corner of your eye to see if he charges or getting too close to smash your head in.
• When Smough uses his charge attack players need to make sure that if they dodge, they dodge just before he swings his hammer upwards because if the player dodges too early, Smough will still reach them. Make sure that Smough is just beginning his upwards swing then dodge, if done correctly he should miss. This is easier if light armor or medium armor with Havel's Ring are equipped.
• Ornstein is weak against fire, using a +5 fire weapon in this fight will be significantly easier. Pyromancies are therefore also quite effective.
• Smough is weak to Lightning in the first form, but gains slight resistance once he transforms, he will share Ornstein's vulnerability to fire though.
• If you want to block, use a high stability shield such as the Greatshield of Artorias.
• The arena has large pillars that will be destroyed by attacks from Smough or Ornstein. 
• The base of each pillar that remains will make good cover to protect against many of their attacks, notably Ornstein's lightning bolts, and Smough's charge-and-hammer-sweep move.
• If you need to heal always try to get behind a pillar first then heal and get out into the battle.
• If you defeated Smough first, Ornstein will pray and absorbs Smough's power, becoming twice his size while retaining his lightning power and speed and gaining the butt attack slam.
• If you defeated Ornstein first, Smough will simply kill the Ornstein (since you still his arm moving) himself with his hammer and imbues it with Ornstein's lightning power.

Strategy

First Phase

Ornstein First
Lock on to Ornstein and always pay attention to where Smough is, when you see Smough do his charging attack make sure a pillar is directly between you (so he hits the pillar) or just run away to the other side of the room. Ornstein will keep closing the distance (keep your shield up or dodge for his lunge attacks) and hit him with a couple hits. Back up and check where Smough is at this point and if he is closing in with another charge repeat the above tactic else just keep baiting Ornstein's attacks. If you are patient you will down Ornstein and Smough will take his power and Phase 2 begins.

Smough First
Keep Ornstein behind Smough as much as possible and land some damage while Smough recovers from an attack and Ornstein has a bit of distance from you.
Remember that Ornstein can hit you through Smough so be very careful. Holding a shield up is highly recommended for this tactic dont hold it up all the time though.
With Ornstein in front of you and Smough to the side, move towards Smough and bait his attack.
If he does any attack except his charge-and-lift attack, dodge it towarsd HIM and hit him a couple times and reposition the bosses again.
This is much harder tactic since the pillar tactic does not really work here, you can still use the pillars for some extra protection.
This may take a few tries to understand their attacks , but eventually when you know when to attack Smough he will go down pretty quick.

Second Phase

"Super" Smough
This second phase is pretty easy all you need to know is that his overhead attack give you ample time to get some hits in.
Dodge / block his attacks and get in a couple hits max.
When he does his butt slam just run away from him and attack him when he is recovering from this attack.

Pyromancy is also very effective against him, dodge or block his attacks and throw Fireballs and Fire Orbs at him.

"Super" Ornstein
Ornstein will be much much bigger, be abit agressive on this phase and keep getting under his legs. His only big dangerous with this attack will be his butt slam attack that he got from Smough.
When you see him crouching abit he will do the butt slam attack run away from it asap don't bother blocking it.
Two-hand your weapon for some extra dmg and just keep attacking his legs. He will keep dodging back, keep running towards and stay close to his legs.
If you get his just find a pillar and heal up and hug him again. Don't be too greedy since he sometimes does a impale that will most likely hit you.
He will sometimes get pissed at your hugging and starts throwing lightning projectiles at you. Hide behind a pillar until he starts his melee attacks again.

If you have any other tips , please let me know!