The safety of transmission lines during power system oscillation has been a long-neglected problem. The conductor temperature exceeds the upper limit of safety during long-term oscillation, which leads to conductor damage or faults. Different operating parameters of transmission lines are closely related to the conductor temperature. Thus, the safety of transmission lines during power system oscillation can be analyzed through the conductor temperature. The time-domain expression of the oscillation current, which is regarded as the cutting point, was initially deduced in this work. Then, the transient thermal balance equation was adopted to calculate the process of conductor temperature increase for the safety analysis of transmission lines during oscillation. Finally, the effectiveness of the proposed method was validated with a case study of typical scenarios. Results show that the response to an increase in conductor temperature is affected mainly by the amplitude of the oscillation current, the oscillation duration, and the meteorological condition of the transmission line. Under severe weather conditions, the conductor temperature exceeds the short-term maximum allowable temperature in a relatively short time. Therefore, a reasonable block time of distance relay or protection against overheating is necessary to ensure the safe operation of transmission lines and to avoid conductor breakdown or cascading faults during power system oscillation.