The term "O katana" is not a common classification for a specific type of Japanese sword. It may refer to an oversized or extra-long katana, but the exact length can vary depending on the individual sword and the specific context in which the term is used.

In general, a traditional katana has an average blade length of around 27 to 30 inches (69 to 76 centimeters). However, the length of an O katana, if used to describe an exceptionally long katana, may exceed these typical dimensions.

An O katana, if considered an oversized or longer version of a standard katana, could have a blade length that extends beyond the usual range. It could range from approximately 31 to 36 inches (79 to 91 centimeters) or even longer. However, it's important to note that swords with significantly longer blades may start to fall into the odachi category, which is characterized by exceptionally long blades.

It's worth mentioning that the specific length of an O katana can vary depending on individual preferences, historical variations, and the purpose for which it was made or used.

In summary, the term "O katana" is not commonly used in the classification of Japanese swords, and its exact length can vary based on the individual sword and context. However, if referring to an oversized or longer katana, it may have a blade length that exceeds the average range of 27 to 30 inches (69 to 76 centimeters), potentially ranging from 31 to 36 inches (79 to 91 centimeters) or longer.