Background: Oral care is a fundamental nurse-led intervention in the critical care setting that provides patient comfort and prevents adverse outcomes in critically ill patients. To date, there has been minimal focus on nurse-focused interventions to improve adherence to oral care regimens in the adult intensive care unit setting. Objectives: The objectives of this study were to (i) identify types and characteristics of interventions to improve oral care adherence amongst critical care nurses and intervention core components, (ii) evaluate the effectiveness of interventions to improve adherence of oral care regimens, and (iii) identify the types of outcome measures used to assess oral care regimen adherence. Design: This is a systematic review in alignment with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. Data sources: Key bibliographic databases and platforms, including Scopus, Cochrane, MEDLINE, CINAHL, Embase, PsycINFO, ProQuest, and Web of Science, were searched for studies published before July 2020. The Joanna Briggs Institute's quality appraisal tool was used to assess risk of bias in included studies. Results: A total of 21 original research studies were identified, of which 18 studies used multifaceted interventions. In accordance with the Joanna Briggs Institute's quality appraisal tools, four of the 20 quasi-experimental studies were rated as high quality. The one randomised control trial was of moderate quality. Outcome measures included oral care adherence behaviours, oral care knowledge, self-reported adherence, and documentation. Improved effectiveness in oral care adherence was reported in 20 studies. Conclusions: Review findings confirm interventions to change behaviours improve oral care adherence. The most effective interventional approach could not be determined owing to heterogeneity in intervention design and outcome measures. Oral care in the intensive care unit is a vital, nurse-led activity that reduces the risk of hospital-acquired infection. It is recommended that future research adopt implementation science methods to ensure stakeholder engagement and feasibility. Systematic Review Registration Number: This review was submitted and subsequently registered on PROSPERO, the International Perspective Register of Systematic Reviews PROSPERO 2019 CRD42019123142.