OASys is the abbreviated term for the Offender Assessment System, used in England and Wales by His Majesty's Prison Service and the National Probation Service to measure the risks and needs of criminal offenders under their supervision. Initially developed in 2001, it was built upon the existing ‘What Works’ evidence base.

OASys is designed to enable a properly trained and qualified individual; often a Probation Officer, to:

• assess how likely an offender is to be re-convicted
• identify and classify offending-related needs, including basic personality characteristics and cognitive behavioural problems
• assess risk of serious harm, risks to the individual and other risks
• assist with management of risk of harm
• links the assessment to the supervision or sentence plan
• indicate the need for further specialist assessments
• measure change during the period of supervision / sentence.

OASys comprises a series of computer-based forms on which clinical evaluations are made by staff of Offenders, and supervision and sentence plans for the forthcoming period of supervision are recorded on a periodic basis - typically every 16 weeks for offenders in the community, and less frequently for imprisoned offenders.

OASys supports the What Works? initiative of the Prison and Probation Services, by providing metrics by which the characteristics of offenders and their offences (inputs) can be analysed alongside information on interventions made (inputs) to the offender, and re-conviction data for offenders (outcomes), in order to enable refinement to be made to interventions (based on a consideration of the relation between inputs and outcomes) as to improve outcomes - in other words, to decrease recidivism rates by ensuring that interventions are as appropriate and purposeful as possible.