What determines the frequency of the oscillator?
Wiki says the ring oscillator frequency is set by the delays of having to charge the gate capacitance of the CMOS stages. The site also says if you increase the CMOS source voltage, the switching time will decrease so the oscillation frequency will increase. I guess this means that less gate capacitance is required to be charged before sufficient current will flow between the source and drain.
You can put a cap in parallel with the gate to ground so that more capacitance is required to be charged. This could possibly decrease oscillation frequency. Lastly you could add more pairs of CMOS stages to decrease frequencty.