Background <p>Older home-dwelling adults with care needs often demonstrate deficient oral hygiene due to cognitive and motor impairments, as evidenced by increased plaque levels, which can compromise oral and general health. Concurrently, dental care utilisation declines with age. Hospitalisation may represent a critical opportunity to address this gap. However, dental assessments are not routinely integrated into multiprofessional inpatient geriatric care. Therefore, the aim of this study was to evaluate the efficacy of an oral hygiene education intervention on the plaque index compared with standard of care (SoC) among older frail acutely ill hospitalised inpatients.</p> Methods <p>In this randomised controlled trial (RCT), hospitalised older inpatients were allocated to an intervention or control group. The intervention group participated in a structured oral hygiene education program including theoretical instruction on oral health and practical assistance in developing a personalised oral hygiene routine during their hospital stay. The control group received standard of care (SoC) on ward without additional oral hygiene support. Dental assessments were conducted at baseline (BL), pre-discharge (FU-1), and three months post-discharge (FU-2). Data on geriatric syndromes, comprehensive geriatric assessment (CGA) and the CGA-based Multidimensional Prognostic Index (MPI) were collected. The oral hygiene intervention and all assessments were conducted by the study dentist.</p> Results <p>Among 36 participants (mean age 79.6 ± 8.1 years), 16 participants of the intervention group (<i>n</i> = 19) achieved a significant reduction in plaque index according to Silness Loe (PI; range 0–3) between BL (mean 2.4 ± 0.5) and FU-1, with an average improvement of 0.5 points (<i>p</i> &lt; 0.001, <i>d</i> = 1.2), while controls (<i>n</i> = 17) showed no improvements (<i>p</i> &gt; 0.05; <i>d</i> = − 0.1). Higher oral health-related quality of life (OHRQoL) scores were linked to better outcomes, while immobility resulted in reduced improvement in oral hygiene. Follow-up at FU-2 was limited by health-related attrition, with only four participants completing the study.</p> Conclusions <p>Hospital-based diagnostics revealed widespread oral health problems of dependent home-dwelling individuals. The intervention improved oral hygiene in this high-risk group during hospitalisation. Integrating oral health diagnostics and interventions into geriatric care plans could address this critical aspect of patient health.</p> Trial registration <p>The study is registered at German Clinical Trials Register (DRKS00027438; date of registration: February 13, 2023).</p>