Oxygen therapy is one of the most widely used medicines. It can also cause harmful effects when used incorrectly and can even be fatal. Aim of the study :was to assess therapeutic effect and complaints among patients receiving supplemental oxygen therapy at Zagazig university hospitals. Subject and methods: Research design: A descriptive design was used. Setting: The study was conducted at all intensive care units and emergency departments at Zagazig University Hospitals, El Sharkia government, Egypt. Subjects: A purposive sample of 30 patients was included in this study. Tools of data collection: Two tools were used: Tool I: A structure interview questionnaire to assess therapeutic effects and patient Tool II: Complaints and standardized dyspnea scale. Results: Study results revealed that oxygen therapy was effective in improving most patients' vital signs, arterial blood gases and dyspnea scale but they had some complaints such as dryness of mouth, nose and eye, ulcers behind the ear, bleeding from the nose and statistically significant relations were found between patients' smoking habit, duration of smoking, oxygen therapy duration and flow rate and therapeutic effect of oxygen therapy. Conclusion: patient's condition was improved with oxygen therapy with some complaints. Recommendations: A close observation for patients receiving oxygen therapy is needed as well as their complaints to save patients life.