Halfway between Potosí and Salar de Uyuni, Oruro is a sad, ugly and uninteresting city. Buses often stop there. Frankly, I wouldn't advise visiting the city unless you absolutely have to.
During my visit, just when I was getting ready to spend the night in the freezing bus station, the wind and the cold forced me to jump on the first bus that stopped there. I was prepared for what awaited me, but never before had I experienced such cold in all my life. It was one of the hardest parts of my trip to Bolivia.
I wasn't even bothered about going there to pass the time. The only interesting thing is the carnival which takes place every year at the end of February. Then, the city is transformed. The daily grey gives way to bright colours. Oruro becomes beautiful and unique.