This oscillator measures overbought/oversold situations. It is said to be an 'upside down' stochastic.

This oscillator is based on the same concept of measuring the last close in relation to the price range over a certain period (below it is 10 days/hours).

%R = 100 x (H10-C) / (H10-L10)

Where C = Last close;  and L10 = lowest low during the chosen period and H10 is the highest high during chosen period.

The %R line is  -  %R = 100 -%K.

The scale in Williams oscillator means that a reading above 20 corresponds to an overbought situation and a reading below 80 corresponds to an oversold situation.