ABSTRACT: This study aims to describe twenty years of early detection, prognosis and preventive care in the Outreach and Support In South-London (OASIS) mental health service for individuals at Clinical High risk of psychosis (CHR-P). The study presents a comprehensive analysis of the 2001- 2020 activity of the OASIS team encompassing core domains: (i) service characteristics, (ii) detection, (iii) prognosis, (iv) treatment and (v) clinical research. The analyses employed descriptive statistics, population-level data, the epidemiological incidence of psychosis, Kaplan Meier failure functions and Greenwood 95% CIs and Electronic Health Records. OASIS is part of the South London and Maudsley (SLaM) NHS trust, the largest European mental health provider, serving a total urban population of 1,358,646 individuals (population aged 16-35: 454,525). Incidence of psychosis in OASIS's catchment area ranges from 58.3 to 71.9 cases per 100,000 person-years, and it is higher than the national average of 41.5 cases per 100,000 person-year. OASIS is a standalone, NHS-funded, multidisciplinary (team leader, consultant and junior psychiatrists, clinical psychologists, mental health professionals), transitional (for those aged 14-35 years) community mental health service with a yearly caseload of 140 CHR-P individuals. OASIS regularly delivers a comprehensive service promotion outreach to several local community organisations. Referrals to OASIS (2366) are made by numerous agencies; about one-third of the referrals eventually met CHR-P criteria. Overall, 600 CHR-P individuals (55.33% males, mean age 22.63 years, white ethnicity 46.44%) have been under the care of the OASIS service: 80.43% met attenuated psychotic symptoms, 18.06% brief and limited intermittent psychotic symptoms and 1.51% genetic risk and deterioration CHR-P criteria. All CHR-P individuals were offered cognitive behavioural therapy and psychosocial support; medications were used depending on individual needs. The cumulative risk of psychosis at ten years was 0.365 (95%CI 0.302-0.437). At six years follow-up, across two-third of individuals non-transitioning to psychosis, 79.24% still displayed some mental health problem, and only 20.75% achieved a complete clinical remission. Research conducted at OASIS encompassed clinical, prognostic, neurobiological and interventional studies and leveraged local, national and international infrastructures; over the past ten years, OASIS-related research attracted about £ 50 million of grant income, with 5,922 citations in the international databases. Future developments may include broadening OASIS to prevent other serious mental disorders beyond psychosis and fostering translational risk prediction and interventional research. With a twenty-years activity, OASIS' cutting-edge quality of preventive care, combined with translational research innovations, consolidated the service as a leading reference model for evidence-based prevention of psychosis worldwide.
