Background: Transitioning a patient from the operating theatre (OT) to the intensive care unit (ICU) is a dynamic and complex process. Handover of the critically ill postoperative patient can contribute to procedural and communication errors. Standardised protocols are means for structuring and improving handover content. Both have been shown to be effective in reducing information omission and improve communication during this transition period. Objectives: The aim of this uncontrolled before and after study was to improve handover processes and communication about the care for critically ill patients transferred from OT to ICU. Methods: Thirty-two OT to ICU handovers (16 before and 16 after implementation) were observed. Using a structured tool, we documented who was present, participated in, and initiated handover during ICU admission. Where and when handover was performed, information provided, distractions and interruptions, and handover duration were also recorded. Unstructured field notes and diagrams provided information on staff interaction. Following implementation, semistructured interviews with 27 participants were conducted to understand participants' perceptions of intervention acceptability and to determine factors influencing intervention implementation and spread. Findings: Following implementation, a “hands-off” approach was observed with fewer technical tasks completed during handover (43.8% before implementation vs 12.5% after implementation) without an increase in handover time. A single, multidisciplinary handover most often led by the anaesthetist was observed after implementation. Despite these improvements, the use of the physical checklist was not observed in practice, and an situation, background, assessment, recommendation (SBAR) format was not followed. Anaesthetists leading the handover did not view the handover checklist as being beneficial to their practice although some nurses were observed to use the checklist as a prompt for additional information.