What rez said is the same as I was told - idea being "O" blood type is the "original" blood type that evolved in homo sapien during the hunter / gatherer era, prior to agriculture. Less "domestic" food, more "wild" food - less grain, fruit, wheat is a no-no, as is oatmeal, dairy, more red meat protein (only a couple eggs / week). Also tend to be hypothyroid. No where is a load of protein required.
Type A and Type AB evolved later during the agriculture period and those types will do better with higher carbs including grains.
That's the $0.05 tour of it. I was also informed of other sort of Eastern ideas as well - to balance out how much "heat" and "cold" was running thru my body. For ex, I used to get "night sweats" (go to bed comfortable, wake up in the middle of the night sweating) - after changing my diet a little, including the juicing, added spirulina to my diet (and some other stuff I can't remember right now) I don't get night sweats any more. Also following a 1 week "detox" period (lots of juiced vegies, miso soup) I was able to eat certain foods and feel "where" they affected my chakras or something like that (e.g. eat jalapenos, get tingly upper arms, etc.) Sounded a little hokey, but seemed to work.
As with any other "diet" - I think there are parts that work and may actually be "true", even if the description makes it sound like old wives' tales or hocus pocus - its got to be based in some knowledge discovered over the years, even if the biochemical reason is not totally understood.
Also to say "it works" is relative - if you go from eating a diet of heavily processed foods, or even a clean diet, but with limited food types (e.g typical competition diet - chicken, steak, fish, protein mix, yams, brown rice, broccoli, spinach, egg whites, flax seed oil) to something that is clean and more varied, you will get some results. Also the part about cutting the simple carbs makes sense as well. And many people are wheat / gluten sensitive and dont' even realize it - they eat lots of wheat bread, etc. because the media tells us its "healthy". And what about cutting out diet Coke, etc? Duh.
Also "O" type is the most common, so these "rules" may appear to work more often.
I try to learn about all the different things I hear about and then study a little, try things and take out what makes sense and what works for me. Personally I think no "diet" is the be all and end all way to go just because its in a best seller. Even a diet that "works" needs to be varied a little to keep the body from getting "content". Change is good.