I have a type O explorer husband who doesn't have that problem, and one of my type O daughters gets constipated very easily. The last year and a half since she was adopted she has always been a very good eater including most veggies whether steamed, baked, or raw. But, earlier on she would only have 2-3 bowel movements a week. I identified the wheat connection as a definite source, but even when she was eating normal (including prunes every day) at home with no gluten sometimes she would still go 48 hours or sometimes even 3 full days before a movement.
Over time it seemed more apparent that the less grains we give her, which include rice, tapioca, teff and millet, the more regular and productive her movements are becoming. I also include a tablespoon of flax or olive oil daily on an empty stomach to alleviate constipation.
Maybe some type O's digestive tracts are prone to more inflammation associated with high grain intake and can't digest them completely and easily. I'm sure there is probably a quoted text someone could find from Dr. D to adequately describe this condition.
I hope this helps.