Lari - Here's a simple test. Get a friend and take a Japanese bokuto out of kashi (evergreen oak) and in graduated increments clash it against your Finnish birch ax handle.
1. Which one dents?
2. Do the dents stay smooth, or do they "feather" - grain lifting up in splinters
3. Which one hurts your hands. Kashi is notable for the way it absorbs shock. Some other woods "ring" in the hands - this means that the shock is not absorbed by the wood.
Re Oak: Not so simple. I recently tested sessile oak (a very common European oak against a kashi bokuto). The sessile oak (also called English brown oak) felt unpleasant in the hands, and broke at about 40% impact. The kashi had no dents (it's lasted me over twenty years). American red oak and American white oak is lousy for weapons. American Live oak, from reports, would be great. Japanese kashi - there are five different varieties, some white, some red. Unfortunately, a fair amount of bokuto from Japan, are, in fact, a Taiwanese red oak, that is not nearly so strong.
Here's a link to my research
on woods for Japanese weapons, to date: I would be very intrigued to find out some good woods in northern Europe as I have a training group in the Netherlands.