People with the blood type O are (supposedly) the descendants of the hunters and gatherers, and are suited to be especially omnivorous, while those with the blood type A are the farmers, and can live on just vegetables.
If type Os lead a strict vegan diet, they can often feel not well and lead to very serious condtitions, but if they even add yoghurt or other animal proteins, they can make their diet much more healthy.
And humans are indeed omnivores, and meat is an essential part of a healthy diet.