The terms "Ō-katana" and "nodachi" are sometimes used interchangeably to refer to large-sized Japanese swords with long blades. However, there can be some variation in how the terms are defined and the distinction between them.

In general, "Ō-katana" translates to "great sword" or "large sword" and typically refers to a long, single-edged sword with a blade length of around 90 centimeters (35 inches) or longer. Ō-katana were primarily used by samurai on foot and were known for their reach and cutting power. They were typically wielded with two hands due to their size and weight.

"Nodachi," on the other hand, is sometimes used specifically to describe a type of Ō-katana with a pronounced curvature along the length of the blade. The term "nodachi" can be translated as "field sword" or "reaping sword." Nodachi blades often had a dramatic curve that facilitated powerful slashing attacks and made them effective against groups of opponents.

However, it's worth noting that the precise definitions and distinctions between Ō-katana and nodachi can vary depending on historical periods, regions, and individual interpretations. Some sources use the terms interchangeably, while others differentiate them based on blade curvature or other characteristics.

In summary, while the terms Ō-katana and nodachi generally refer to large-sized Japanese swords with long blades, the specific definitions and distinctions between them can vary. They are often used to describe similar types of swords that were favored by samurai warriors in ancient Japan.