Ouro Preto is not really a walkable city. It is small and compact enough, but the hills are dauntingly steep. Many of the hills are steeper than an average staircase and they are a lot longer. There are also uneven pavements and big steps up & down. I don’t think of myself as lazy and I am in pretty good condition, but this is just not a place for a nice stroll.
On the plus side, you get a good workout just going from the hotel to a restaurant.
It is not an easier place for cars. The streets are narrow with lots of sharp turns and the steep hills are also difficult for cars, as you can see in the pictures. Above is the monument to Tiradentes, who rebelled against the Portuguese, was killed in a nasty way and became a hero-martyr.