In this picture you can see a nice view of downtown Ouro Preto.
Ouro Preto is a couple of hours by bus from Belo Horizonte. This latter town has an International Airport, with frequent flights from Rio de Janeiro and Sao Paulo.
The town has a network of public buses. But I do not think it is necessary to take buses in a city where the main tourist attractions are accessible on foot. The town also has a fleet of taxis.