Dragon Slayer Ornstein and Executioner Smough

Dragon Slayer Ornstein and Executioner Smough are a pair of bosses found in Anor Londo. Players must defeat them in order to reach Gwynevere, Princess of Sunlight.

Alongside Hawkeye Gough, Artorias the Abysswalker and Lord's Blade Ciaran (all of whom appear in the Artorias of the Abyss DLC), Dragonslayer Ornstein was one of Gwyn, Lord of Cinder's four knights.

Summoning
It is possible to summon Solaire of Astora as a white phantom to help out in the fight. His summon sign can be found at the top of the flight of stairs just before the fog door.

Strategies
The player must fight both at once. When one is killed the remaining one absorbs their dead comrade and receives an HP, damage and resistance boost (Ornstein grows in size while Smough becomes electricfied), as well as recovering all lost HP. Having a summon can make the fight significantly easier, as this will often cause them to fight separately, making them unable to help each other.

Killing Ornstein first can make the fight easier to win, since Smough has more weaknessess than his partner.

Swinging Combo
Ornstein swings his spear up to four times in a row. Can be blocked.

Jumping Attack
Ornstein jumps up, and then either stabs at the player, or smashes his spear into the ground where the player is standing. Both deal the same amount of damage, and it can be blocked.

Gliding Stab
Ornstein glides towards the player, and then stabs with his spear. Can be blocked or dodged, and a counterattack is possible to achieve.

Lightning Bonzai Drop
Only used if Smough was killed first. Ornstein jumps up, pauses, then falls down to the ground, causing a large field of lightning to spread from the point of impact. Cannot be blocked, and can only be dodged by running away from the point of impact.

Lightning Bolt
Ornstein charges up a lightning bolt, then fires it at the player. Can be dodged by running to the side, or hiding behind one of the many pillars in the arena.

Lightning Impale
Ornstein only uses this attack when Smough is dead. He charges his spear with lightning, then stabs out at the player. If it hits an unblocked player, Ornstein then raises the player, and releases the charged lightning. Deals heavy damage.

Joust (Charge Attack)
Smough charges at the player, then performs a diagonal upwards slash when he stops. Deals heavy damage, especially when in Lightning Form. The attack can be avoided by running directly away because the attack can easily track any sideways movement. Blocking this attack is difficult as the charge hits many times which may break the player's guard.

Swing
Smough swings his weapon. It is possible to roll under this attack and start attacking before Smough has time to recover. Blocking this attack may use a large amount of stamina so it is better to try and roll under it.

Hammer
Smough slowly raises his weapon and then hammers the ground in front of him.

Jumping Hammer
Smough jumps into the air and then hammers the ground in front of himself. Can be avoided by rolling under Smough while he is airborne.

Banzai Drop
Smough slowly flies into the air and then crashes into the ground below. Smough takes a while to recover after using this attack. The Lightning Form of Banzai Drop also fires out lightning when Smough lands.

Trivia

 * Dragon Slayer Ornstein is presumed to be the Captain of Gwyn's Four Knights.