Background: Preferences for end-of-life care for patients with cancer are often overlooked, burdening patients’ families with decision-making. Advanced care planning (ACP) is a multidisciplinary approach that supports engaging patients with progressive cancer in their treatment course and providing them with a particular medical care based on their preferences. However, there is a lack of studies that assess the oncology physicians and nurses' knowledge, attitudes, and practice behaviour about ACP in Middle Eastern countries. Purpose: This study aimed to assess the knowledge, attitudes, and practice behaviours of oncology physicians and nurses regarding ACP for patients with cancer in Oman. Methods: A cross-sectional, descriptive correlational design using standardized questionnaires measuring knowledge, attitudes, and practice behaviours of oncology physicians and nurses regarding ACP for patients with cancer was used. A convenience sample of 296 oncology physicians and nurses were recruited from three tertiary hospitals in Muscat. Both descriptive statistics (i.e., mean, SD, frequency, and percentages) and inferential statistics (Pearson Correlation Coefficient and Independent samples t-tests) were used to analyse the data. Results: The participants consisted of physicians (28.7%, n = 85) and nurses (71.3%, n = 211). The majority of participants were female (73.4%, n = 218). The findings showed that 91.9% (n = 273) of the participants had knowledge about ACP that was less than 60%, with a mean total score of 8.25 out of 14 (SD = 2.73), indicating poor knowledge about ACP. Although, the participants had positive attitudes toward ACP, with a mean total score of 74.19 out of 105 (SD = 11.61), their practice behaviours regarding ACP were poor, with a mean total score of 11.25 out of 20 (SD = 3.91). There was significant positive correlations between knowledge and attitude, knowledge and practice behaviours, and attitudes and practice behaviours regarding ACP (r = 0.238, p = 0.001; r = 1.046, p = 0.001; r = 0.294, p = 0.001 respectively). The independent samples t-tests indicated that physicians had higher knowledge and more positive attitudes about ACP (M = 9.07, M = 81.86 respectively) compared to nurses (M = 7.87; M = 70.89 respectively) (p 0.001). However, there was no significant difference between physicians and nurses in their practice behaviours toward ACP (p = 0.077). The participants with a higher education level exhibited greater knowledge and more positive attitudes towards ACP (p 0.001). The major barriers to implementing ACP for patients with cancer were found to be a lack of knowledge (75.1%) and communication skills in handling ACP (57.6%). Conclusion: The study highlights the poor knowledge and limited practice behaviours regarding ACP among oncology physicians and nurses in Oman. Improving knowledge about ACP can lead to better practice behaviours and more positive attitudes toward ACP. There is need for educational programs and campaigns targeting healthcare professionals, policymakers, and the general public to enhance their knowledge and promote good practices related to ACP. Hence, ACP courses should be included in the medical and nursing curricula, covering ACP communication skills. The findings have implications for initiating national ACP guidelines to support the implementation of ACP in Oman.