The concentration of dissolved oxygen in water at pressures in the range from 0.1 to 1 atm was determined by Winkler's method in order to elucidate previous results on the sonochemical oxidation of potassium iodide, where the reaction rate slowed down at some critical pressures that depended on the ambient gas. The concentration was found to be nearly proportional to the pressure, but the concentration relative to saturation varied depending on the method of pressure control. Further evidence for different dynamics below and above the critical concentration was that the tone of cavitation noise changed around there. Assuming that the density of cavitations is proportional to the gas concentration, it is suggested that the cavitation bubbles are relatively stable in the low-density regime, since their mutual interaction is weak. Above some critical density, however, the bubbles may interfere with one another, inducing the implosion at a moment earlier than that for the minimum bubble size achieved in the low-density regime. This high-density behavior results in the decrease of the reaction rate, and should usually be seen in the conventional sonochemistry, where high-power ultrasounds are employed. It is also suggested that, as the pressure is varied, single bubble sonoluminescence may be similarly affected.