The frequency of oscillation is the measure of how often a wave peaks in a given time frame. Lots of phenomena occur in waves. Ripples on a pond, sound and other vibrations are mathematically described in terms of waves.
A typical waveform has a peak and a valley -- also known as a crest and trough -- and repeats the peak-and-valley phenomenon over and over again at a regular interval. The wavelength is a measure of the distance from one peak to the next and is necessary for understanding and describing the frequency.
Determine the wavelength of the waves in question. This is the measure from one peak to the next. You can determine the approximate wavelength of light based on its color. To determine the wavelength of sound waves, you may need an oscilloscope.
Calculate the frequency of oscillation by dividing the speed of travel by the wavelength.
TL;DR (Too Long; Didn't Read)
No matter what type of oscillating system you are working with, the frequency of oscillation is always the speed that the waves are traveling divided by the wavelength, but determining a system’s speed and wavelength may be more difficult depending on the type and complexity of the system.