The viscosity of oil and natural gas is a key property when developing petroleum reservoirs and an important parameter for the transportation of produced fluids. In Petroleum Industry different viscosity measurement principles are applied for the experimental determination of the oil and gas viscosity, e. g. the Rolling Ball Viscometer principle. Therefore a Rolling Ball Viscometer was calibrated within this thesis work to be in the position to measure oil viscosity under reservoir conditions. The measurement of oil viscosity becomes more and more difficult as viscosity increases. In many cases an accurate measurement of heavy oil viscosity is not possible. However, the viscosity can be predicted by using correlation models which are set up on certain oils. Consequently, a distinct correlation model can not be used to predict oil viscosity in any case. In the frame of this thesis work, correlation models have been developed for saturated and for under-saturated Vienna Basin Oils. These models were tested by predicting experimental data and were found to be more accurate than the correlation models used until now.