An acronym for main refinancing operation; an open market operation (OMO) that entails providing one week liquidity to the banking system. It is a regular liquidity-providing reverse transactions generally with a frequency and maturity of one week. The interest paid for such short-term loans is known as MRO rate.

In particular, MROs are carried out by central banks in Europe, the euro area, (that all form part of a unified monetary policy imposed and implemented by the European Central Bank, ECB), on the basis of standard tenders, according to a calendar published by the ECB.

MRO is an official interest rate that forms one of the bases for interest rates on the money market.