is it still as good as eating cooked oatmeal?
No it is not the same.
In other words, does the cutting of the oatmeal by the blades of the blender move it into the "flour" category?
No, it is not in the flour category but there are degrees of influence of the grinding process and the less the better. The reason is, the more you grind up food, the more of the surface area you expose, the quicker it is absorbed the more likely it can effect blood sugar and insulin and the more of it you can consume.
--the less blending, processing, etc we do and the more we get to use out teeth and facial muscles in chewing, the better. Chewing is an important part of the digestive process which can not be replaces by a blender.
--In addition, if someone was watching their blood sugars, or their triglycerides, I would rather see them consume their fruits/foods more whole and less blended/processed.
-- Studies have shown that people can eat more when their food is blended up (ie smoothie), so if someone was trying to lose weight, consuming the whole food would be better.
You may want to read this discussion we had on blending and processing of food.