Background. Quality of life (QoL) is an important measure to understand the effect of diseases on patients' daily living. Yet, the research on QoL among patients with chronic diseases in Oman is scarce. This study aimed to assess the level and determine the predictors of QoL among patients with chronic diseases in Oman. Methods. A cross-sectional correlational descriptive was run among 340 patients with chronic obstructive pulmonary disease (COPD), chronic heart failure, and/or end-stage renal disease. Results. Participants' mean age was 60.6 years (SD ± 14.4) and the majority were males (63.5%). The highest mean scores were emotional well-being (mean = 85.2, SD = ±11.2) and role limitations due to personal or emotional problems (mean = 83.2, SD = ±35.8). In addition, COPD patients reported the lowest scores on the QoL questionnaire. Having a high score on the Karnofsky Performance Status Scale (KPSS) and being married predicted a higher score in the mental and physical domains of QoL, while having a higher total symptom number and COPD predicted a lower score in the mental domain. Older age, higher total symptom number, and a higher distress level predicted a lower score on the physical domain of QoL. Conclusion. The results call for a need to improve the QoL among the patients with chronic disease, including symptom management plans, self-management programmes, and training to enhance their own perception of QoL. Furthermore, symptom management plans are highly needed.