Open Interest is the number of open contracts of a
given future or option contract. An open contract can be a long or short
contract that has not been exercised, closed out, or allowed to expire.
Open interest is really more of a data field than an indicator.
A fact that is sometimes overlooked is that a
futures contract always involves a buyer and a seller. This means that
one unit of open interest always represents two people, a buyer and a
seller.
Open interest increases when a buyer and seller
create a new contract. This happens when the buyer initiates a long
position and the seller initiates a short position. Open interest
decreases when the buyer and seller liquidate existing contracts. This
happens when the buyer is selling an existing long position and the
seller is covering an existing short position.
Interpretation
By itself, open interest only shows the liquidity of
a specific contract or market. However, combining volume analysis with
open interest sometimes provides subtle clues to the flow of money in
and out of the market:
Rising volume and rising open interest confirm
the direction of the current trend.
Falling volume and falling open interest signal
that an end to the current trend may be imminent.
Example
The following chart shows Copper, open interest (the
solid line), and volume (the dotted line).
The open interest is for all copper contracts, not
just the current contract.
I drew a trendline ("A") when both open interest and
volume were increasing. This confirmed the upward trend of prices as
shown by the trendline ("B").
I then drew a vertical line ("C") when open interest
and volume began to diverge. From this point, volume continued to
increase while open interest decreased sharply. This warned of an end to
the rising trend.