An irreversible cycle model of the Otto heat engine is established, in which the temperature-dependent heat capacities of the working fluid, the irreversibilities resulting from the nonisentropic compression and expansion processes, and heat leak losses through the cylinder wall are taken into account. The adiabatic equation of ideal gases with the temperature-dependent heat capacity is strictly deduced without using the additional approximation condition in the relevant literature and used to analyze the performance of the Otto heat engine. Expressions for the work output and efficiency of the cycle are derived by introducing the compression ratio of two isochoric processes. The performance characteristic curves of the Otto heat engine are presented for a set of given parameters. The optimum criteria of some important parameters such as the work output, efficiency, compression ratio, and temperatures of the working fluid are given. Moreover the influence of the compression and expansion efficiencies, the variable heat capacities, the heat leak, and other parameters on the performance of the cycle is discussed in detail. The results obtained are novel and general, from which some relevant conclusions in literature may be directly derived. This work may provide a significant guidance for the performance improvement and optimal design of the Otto heat engine.