This paper presents the results of numerical simulations of organic Rankine cycles (ORCs) for low-grade heat source temperature in the range of 373–653 K for 20 working fluids. The goal of the study is to determine the importance of the level of the source temperature in the selection of working fluids and operation conditions. As a performance indicator, a function of the net power, the internal and external exergy efficiencies, and the exergy efficiency of the evaporator is used. The results show that there is a strong correlation between the level of the source temperature and the critical temperature, between the expander inlet temperature and the source temperature, and between the expander inlet pressure and the critical pressure of the working fluids tested. The study shows that once the source temperature level is known, it is possible to select the working fluid and the conditions at the expander entrance.