len - formula specifying a number of bars
avg - formula specifying a number of bars
The Stochastic Oscillator measures the position of a value in a range of similar values, with optional smoothing.
STOC(len, avg) could also be expressed as 100 * Avg((C - Lowest(C, len)) / (Highest(C, len) - Lowest(C, len)), avg).
In case you're looking for the "Williams %R" indicator, that is simply -1 * (100 - STOC(len, 1)).