The main difference between an odachi and a katana lies in their size and length. The odachi, also known as a nodachi, is an extra-long Japanese sword that typically has a blade length of over 90 centimeters (35 inches). In contrast, a katana is a standard-sized Japanese sword with a blade length typically ranging from 60 to 80 centimeters (24 to 31 inches).

Here are some key distinctions between the two:

1. Size and Length: The odachi is significantly longer than the katana. Its extended length provides greater reach and potential striking power, making it suitable for engaging opponents from a distance. The katana, being shorter, offers more maneuverability and agility in close-quarters combat.
2. Two-Handed Grip: Due to its size, the odachi is usually wielded with a two-handed grip. The longer hilt of the odachi allows for a two-handed grip, providing better control and stability when swinging the sword. In contrast, the katana is typically wielded with a one-handed grip, although it can also be used with two hands for certain techniques.
3. Combat Style and Usage: The odachi was historically used by foot soldiers and was particularly effective in open-field battles or against armored opponents. Its length and weight allowed for powerful strikes and increased leverage. The katana, as the iconic sword of the samurai, was versatile and suitable for various combat scenarios, including duels, skirmishes, and indoor engagements.
4. Handling and Practicality: Due to its larger size and weight, the odachi can be more challenging to handle compared to the katana. The katana's design, with its balance and versatility, allows for quick strikes, agile movements, and precise cuts. The katana's size and handling make it more practical for day-to-day use and carrying.
5. Historical Significance: Both the odachi and the katana hold historical significance in Japanese swordsmanship and samurai culture. The odachi was primarily used in earlier periods of Japanese history, while the katana gained prominence during the feudal era.

It's important to note that the odachi is a rarer and less commonly seen sword compared to the katana. The size and length of the odachi make it more specialized and limited in its usage. The katana, with its balanced design and versatility, remains one of the most iconic and widely recognized swords in Japanese history.