Open market operation
An open market operation (OMO) is an activity by a central bank to give (or take) liquidity in its currency to (or from) a bank or a group of banks. The central bank can either buy or sell government bonds (or other financial assets) in the open market (this is where the name was historically derived from) or, in what is now mostly the preferred solution, enter into a repo or secured lending transaction with a commercial bank: the central bank gives the money as a deposit for a defined period and synchronously takes an eligible asset as collateral.
Central banks usually use OMO as the primary means of implementing monetary policy. The usual aim of open market operations is—aside from supplying commercial banks with liquidity and sometimes taking surplus liquidity from commercial banks—to manipulate the short-term interest rate and the supply of base money in an economy, and thus indirectly control the total money supply, in effect expanding money or contracting the money supply. This involves meeting the demand of base money at the target interest rate by buying and selling government securities, or other financial instruments. Monetary targets, such as inflation, interest rates, or exchange rates, are used to guide this implementation.
In the post-crisis economy, conventional short-term open market operations have been superseded by major central banks by quantitative easing (QE) programmes. QE are technically similar open-market operations, but entail a pre-commitment of the central bank to conduct purchases to a pre-defined large volume and for a pre-defined period of time. Under QE, central banks typically purchase riskier and longer-term securities such as long maturity sovereign bonds and even corporate bonds.