On Balance Volume is an oscillator developed by Joe Granville, which relates price movement to volume. When the closing price is higher than the previous day the volume is added to the previous OBV value, and when the price is lower the volume is subtracted. This gives a cumulative value to measure positive and negative volume flow.
This is a companion discussion topic for the original entry at https://www.optuma.com/kb/optuma/tools/volume/on-balance-volume--obv-