Nodachi/Odachi swords are truly awesome cold weapon. It is a large, very large two-handed sword which devastate any enemy near the warrior using it. Just look at its pictures:
Nodachi have the same general appearance and design of a tachi though they are significantly longer. The nodachi was carried by foot soldiers and was designed as a weapon for war versus cavalry and open field engagements. Nodachi were generally used on open battlefields as their length made their use indoors or close quarters difficult. They were an effective weapon against cavalry, though they were not commonly used. Nodachi were infrequently used for several reasons:
The blade was more difficult to forge compared to a normal-sized sword
The nodachi required greater strength to properly wield
Weapons such as the naginata or nagamaki were arguably more effective for the same role on the battlefield.
During times of peace the sword was worn slung across the back as a symbol of status. This is distinctive because most Japanese swords such as the katana, wakizashi, and tachi were worn at the waist or belt; however it was not "drawn" from the back. The nodachi was more difficult to wield due to its abnormal size and weight, but like any weapon, could be extremely deadly if the warrior wielding it was skilled. The size of the blade made the nodachi a fearsome weapon when wielded by a skilled warrior. The length of the nodachi's hilt varied between twelve to thirteen inches (30 to 33 centimeters). Its cutting capability and range exceeded that of a katana, due to its weight and size. Legend says that a nodachi could cleave a warrior and his horse in half with a single blow.
In some Chinese martial arts, Pa Kua Chang being perhaps the best known example, oversized weapons are used for training purposes in order to condition the martial artist to handle a normal-sized weapon more efficiently (as is the case in Japanese martial arts with the suburito, a heavy wooden sword).
The Kage-ryu one of the very rare ryuha remaining that trains in the use of the Japanese long-sword (which they call choken).
The definition of Nodachi
The direct translation of "Odachi" is big thick sword. Often mistaken for Nodachi which translates to field sword. The term Nodachi is generally misused for Odachi.
The character for "O" means big or great. The characters for "DA" and "CHI" are the same as "TACHI", the older style of sword/mounts that predate the Katana (the "CHI" is the same character as Katana and the "TO" in Nihonto).
I think that to qualify as a Nodachi the sword in question must have a blade length of 3 shaku (90.9 cm) or more, however, as with most terms in Japanese sword arts, there is no exact definition of the size of a Nodachi. Generally speaking, swords that are longer than normal* ones are classified as Nodachi.
*The normal length of swords varied slightly depending on the period. During the Edo era, the average length of a sword was about 66 - 70 cms, however during the Heian and Kamakura era it was around 80 cm. Lengths varied during different eras because the methods of use varied also. During the Heian and Kamakura era Samurai rode on horseback which required a longer blade (about 80 cm). Samurai during the Edo era didn't usually ride horses, so a shorter (about 66 -70 cm) blade was more useful.
The history of Nodachi
It is believed that Odachi first appeared in the 5th Century. This has been proven by the fact that a 117.0 cm sword from the 5th Century was unearthed from an old mound in Kumamoto. A 137.9 cm sword was unearthed from a mound in Tochigi and dated to the 5th Century.
It was believed that some such swords were used by the Gods of Japanese mythology, however, most of the swords discovered so far have only dated to the 5th Century.
For example, in Japanese mythology the sword named "Hutsunomi-tamano-tsurugi" (in Kashima Shrine, Ibaraki) was believed to have been the sword that was given to the Emperor by a God to end a revolt. Now, research has show that the sword was made during the Heian era and is not the sword from the myth.
It seems the sword was an imitation piece, based on the sword used in mythology. This is the reason why some swords are believed to have been used by Gods. Another one of the reasons why people believed that the sword was used by Gods was due to the appearance of the sword. If the sword is large and awesome, it looks like the property of a God.
The purpose of Nodachi
The purpose of Nodachi can be categorized as follows:
a) As an offering to a Shrine or Gods. Some Nodachi were dedicated with prayer to win a war, others were placed in Shrines as legendary swords from mythology.
b) As a weapon. From explanations in old texts, such as "Heike-monogatari, Taihei-ki" tell us that Odachi were used by soldiers during battles.
c) As a symbol for an army. Some Nodachi are too long for practical use. They cannot be used in a battle but it is said that they could have been used as a symbol of an army, such as flags and spears. Further research is needed to confirm this idea.
d) As a trend during a certain period. Some swords were also used for ceremonies.
e) To show the swordsmith's skill.
The production of Nodachi
Nodachi are very difficult to produce. The requirements to make a good Nodachi are as follows:
a) A lot of steel is needed to produce a Nodachi and it takes longer to make than a normal sword. However to make a good Nodachi it is important to hammer the steel quickly. This requires great skill from the swordsmith.
b) A Nodachi is made with teamwork. Perfect teamwork is required to make a good one.
c) More skill is required in quenching and tempering a Nodachi than a normal sword.
d) Special facilities are required. For example the quenching tank must be bigger than that used for normal swords.
e) The method of polishing is different. Odachi need to be hung from the ceiling or placed in a stationary position to be polished, unlike normal swords which are moved over the polishing stones.
How to use Nodachi
Nodachi that were used as weapons were too long for Samurai to carry on their waists like normal swords. There were two methods in which Odachi could be carried. One method was to carry it on your back. This was impractical however, as it was impossible for the Samurai carrying the sword to draw it quickly. The other method was simply to carry the Nodachi by hand. The trend during the Muromachi era was for the Samurai carrying the Nodachi to have a follower to help him draw it.
Nodachi swordplay styles focused on downward chops and different wields to that of normal swords.
The use of Nodachi in early times
The Nodachi's importance died off after the Osaka-Natsuno-Jin war of 1615 (battle between Ieyasu Tokugawa and Mitsunari Ishida). Since then it has been used more as a ceremonial piece.
The two main reasons for losing popularity are:
1. Battles in fields did not occur after 1615.
2. The Bakuhu government set a law which prohibited holding swords above a set length (in Genwa 3 , Kwanei 3  and Shoho2 ).
After the law was put into practice, Nodachi were cut down to the shorter legal size. This is one of the reasons why Nodachi are so rare.
Nodachi were no longer of practical use, but were still made as offerings to Shinto shrines. This became their main purpose. Due to the amount of skill required to make one it was considered that their awesome appearance was suitable for praying to the Gods.http://japantrip.tripod.com/nodachi/nodachi.html