It stands for order imbalance; a situation that arises when an exchange adjusts its quotes in the opposite direction of arriving trades. This instigates a significant, negative autocorrelation in returns, whose proportion and effect depend on the difference between buyer and seller-initiated trades (i.e., order imbalance (OIB)).

The order imbalance is the number of buyer-initiated trades minus the number of seller-initiated trades. Buyer- and seller-initiated trades are identified, in practice, by matching transactions with changes in limit orders. For example, a precursor to buyer-initiated trades is the behavior of ask orders in the order book that is reflected in a change with the equal quantity at the same price and the same time.

Under this condition (order imbalance), prices tend to reverse, resulting in the so-called the OIB-reversal effect.