I don't see much practical use in riding a bull instead of a horse, they are slower, for one, with an average top speed of just around 40km/h compared to a horse which has a top speed of 88km/h, and are considerably dumber than horses.
Not to mention the most crippling problem, their eyesight.
- Field of Vision-
When cattle have their heads down to graze, they can see almost 360 degrees. As they raise their heads, the sides of their bodies create a blind spot behind them. This panoramic vision allows them to see in all directions without moving their heads, but they have binocular vision only for an angle of about 25 to 50 degrees in front of them. Binocular vision is like ours: Both eyes focus on an object, allowing the perception of depth, speed, and distance. Their monocular vision to the side allows them to see movement, but it is not sharply focused like human vision.
- Depth Perception-Cows have slit-shaped pupils and weak eye muscles, which means they cannot focus quickly. They have poor depth perception because they have limited vertical vision of about 60 degrees, compared with around 140 degrees for humans. Because of this, they do not like to walk across shadows as they cannot tell if a shadow is a hole or ditch; they will stop moving forward and lower their heads to the ground to check it out.
If your character is heading into battles or even war, the bull would not be as reliable as a war horse which has been bred for centuries for the purpose of carrying soldiers.
Now regarding the orc. I'm going to make assumptions here and say that the half-orc would be around half of what an orc would weigh. According to the wow wiki (which I used as my reference) Orcs weigh between 226 - 652 lb. I'm going to assume the worst and go with the upper bound of 652 lb, which divided by two equals 326 lb.
I am also going to assume that the Orc is wearing some sort of armour, and I'm going to use common medieval iron plate armour as my reference. With some searching, the average weight for full plate armour I found was between 33-55lbs, and since I'm assuming that the half-orc is bigger than a human, maybe upper 6 to 7+ feet tall, I'm going to add a few extra pounds to the upper bound, which comes to around 60 lb.
(I am not going to take into consideration any extra armour/weapons/food the half-orc is carrying with him, for simplicity's sake.)
Summing it up, the total weight this bull would have to carry would be roughly 386 lb on its back. This may seem a lot, but apparently, not quite.
According to this page, the Barns-ford Ferny, a Charolais bull, weighing at around 4,000 lbs could easily pull 4,000 pounds at a walking pace.
Taking this fact into consideration, it seems very possible for a bull to carry the orc along with iron plate armour, easily, whereas horses are only really supposed to carry no more than 20% of their own weight. (The heaviest horse in history was the shire horse, which weighed 3,360 lb, 20% of which still isn't enough my estimations for your half-orc)
And regarding the history question- No.
Summary: It is possible for the bull to at least carry your half-orc into battles or just to a town etc. equipped with all his miscellaneous wares. It is, however, not recommended to use a bull for battle, as they are slower, dumber, and generally more unreliable to horses.