Relaxation oscillators use two principles to work. First is hysteresis where two activation thresholds are used to differentiate the switch on input level of the op amp and the switch off level. The other principle used is integration through an RC chain. This sets a frequency reference for the op amp to use to move between the two threshold levels. The RC chain has a given time constant at which the capacitor can charge/discharge through the chosen resistor to reach the two threshold levels - this alternation between the two threshold levels sets the frequency of oscillation. The hysteresis can also be used to set a timebase for the Oscillator. The op amp finds role as an open loop gain (ideally infinite) level switcher to feed the integration chain and make the oscillation process work.
