The Office for National Statistics (ONS; Welsh: Swyddfa Ystadegau Gwladol) is the executive office of the UK Statistics Authority, a non-ministerial department which reports directly to the UK Parliament.

The ONS is responsible for the collection and publication of statistics related to the economy, population and society of the United Kingdom; responsibility for some areas of statistics in Scotland, Northern Ireland and Wales is devolved to the devolved governments for those areas. The ONS functions as the executive office of the National Statistician, who is also the UK Statistics Authority's Chief Executive and principal statistical adviser to the UK's National Statistics Institute, and the 'Head Office' of the Government Statistical Service (GSS). Its main office is in Newport near the United Kingdom Intellectual Property Office and Tredegar House, but another significant office is in Titchfield in Hampshire, and a small office is in London. ONS co-ordinates data collection with the respective bodies in Northern Ireland and Scotland, namely NISRA and NRS.

The ONS was formed on 1 April 1996 by the merger of the Central Statistical Office (CSO) and the Office of Population Censuses and Surveys (OPCS). Following the Statistics and Registration Service Act 2007, the United Kingdom Statistics Authority became a non-ministerial department on 1 April 2008.

ONS produces and publishes a wide range of the information about the United Kingdom that can be used for social and economic policy-making as well as painting a portrait of the country as its population evolves over time. This is often produced in ways that make comparison with other societies and economies possible. Much of the data on which policy-makers depend is produced by ONS through a combination of a decennial population census, samples and surveys and analysis of data generated by businesses and organisations such as the National Health Service and the register of births, marriages and deaths. Its publications, and analyses by other users based on its published data, are reported and discussed daily in the media as the basis for the public understanding of the country in which they live.

The reliance on some of these data by government (both local and national) makes ONS material central to debates about the determination of priorities, the allocation of resources and for decisions on interest rates or borrowing. The complexity and degree and speed of change in the society, combined with the challenge of measuring some of these (e.g. in relation to longevity, migration or illness patterns or fine movements in inflation or other aspects of national accounts) give rise to periodic debates about some of its indicators and portrayals. Many of these rely on sources which are outside ONS, while some of its own sources need to be supplemented, for example between censuses, by updated but less rigorously obtained information from other sources. Consequently, unexpected or incomplete data or occasional errors or disputes about its analysis can also attract considerable attention. And this helps to show data that is complete.

ONS data can also be used in epidemiologic studies such as survival analysis.

Gordon Brown, then Chancellor of the Exchequer, announced on 28 November 2005, that the government intended to publish plans in early 2006 to legislate that the ONS and the statistics it generates are independent of government on a model based on the independence of the Monetary Policy Committee of the Bank of England. This was originally a 1997 Labour manifesto commitment and was also the policy of the Liberal Democrat and Conservative parties. Such independence was also sought by the Royal Statistical Society and the Statistics Commission. The National Statistician would be directly accountable to Parliament through a more widely constituted independent governing Statistics Board. The ONS would be a non-ministerial government department so that the staff, including the Director, would remain as civil servants but without being under direct ministerial control. The then National Statistician, Dame Karen Dunnell, stated that legislation would help improve public trust in official statistics (although the ONS already acted independently, as per its own published guidelines, the National Statistics Code of Practice, which set out the key principles and standards that official statisticians, including those in other parts of the government statistical service, were expected to follow and uphold).

The details of the plans for independence were considered in Parliament during the 2006/2007 session and resulted in the Statistics and Registration Service Act 2007. In July 2007, Sir Michael Scholar was nominated by the government to be the three-day-a-week non-executive chairman of the Statistics Board which, with the intention of re-establishing faith in the integrity of government statistics, was to take on statutory responsibility for oversight of UK statistics in April 2008 and oversee the Office for National Statistics; also having a duty to assess all UK government statistics. Following Gordon Brown's announcement of new constitutional arrangements for public appointments, Sir Michael also became, on 18 July, the first such nominee to appear before the House of Commons Treasury Committee and to have his nomination subject to confirmation by the House. On 7 February 2008, following the first meeting of the shadow board, it was announced that it would be known as the UK Statistics Authority (UKSA).