The Red Oak Wood used for Japanese Bokken, also called 'Akagashi', greatly differs from the Red Oak Wood referred to when speaking about North American Bokken. 
While the North American Oak and Japanese Oak are related, the Japanese Oak tree is evergreen, due to this it does not have a conspicuous open grain like the American White and Red Oak. 
The open grain structure, of the American Oak wood produces soft areas which are more prone to impact damage. 
Akagashi on the other hand has a structure that is uniformly hard, has excellent dent resistance and has better impact strength than American Oaks. 
Nowadays Akagashi is a quite rare and expensive even in Japan, and is mostly used for making high quality Bokken.