This article is from the Glossary of Technical Analysis Terms.
OBV is one of the most popular volume indicators and was developed by Joseph Granville. Constructing an OBV line is very simple: The total volume for each day is assigned a positive or negative value depending on whether prices closed higher or lower that day. A higher close results in the volume for that day to get a positive value, while a lower close results in negative value. A running total is kept by adding or subtracting each day's volume based on the direction of the close. The direction of the OBV line is the thing to watch, not the actual volume numbers.
C=Today's Close CP=Yesterday's Close V=Today's Volume