In applications of atmospheric-pressure plasma jets, due to the limitations of common power supplies, the power coupled to the plasma usually varies, which affects the production of reactive species. Here we maintain the power input constant to gain insights into the effects of variations of driving voltage and frequency on the generation of OH reactive species, and give practical recommendations for energy efficient plasma processing. The argon plasma jet generated by AC power is studied under different voltage and frequency under the custom-adjusted constant-power conditions, and optical spectroscopic measurements are carried out. The results show that under the same power, the gas temperature decrease with the increase of frequency from 380 K to 310 K, while the electron density stays the same at about 2.13 × 1015 cm3. The electron excitation temperature decreases from 4689 K to 4176 K with the increase of frequency. The density of ozone rose from 8.65 × 1018 m−3 to 3.20 × 1020 m−3. Though the power is constant, the changes of the plasma parameters affect the density of OH radicals. The latter rises when the driving voltage is increased. These findings provide guiding principles for the development of effective and energy efficient plasma processes.