Background: Supplemental oxygen therapy (SOT) is a lifesaving drug when used appropriately and cause harm if used inappropriately. Administering SOT has an essential role in preventing and managing hypoxemia in both acute and chronic conditions. The aim of the study was to evaluate the effect of nursing guidelines regarding safe and effective practices of supplemental oxygen therapy among critically care patients. Research design: Quasi- Experimental research design (pre- post test) was utilized to achieve the aim of the study. Setting: This study was conducted at Intensive Care Unit (ICU) in Benha University Hospital. Sample: Convenient sample of (60) nurse from both sex who working in ICU during the time of data collection and agree to participate in this study. Tools of data collection: Two tools were used, Tool I: Self-administered questionnaire which consisted of two parts to assess A) Nurses' demographic data. B) Nurses' knowledge questioners about safe and effective administration of supplemental oxygen therapy and Tool II: Observational checklist for nurses' practice regarding safe and effective supplemental oxygen therapy administration. Results: The study revealed that nurses' knowledge and practice regarding safe and effective supplemental oxygen therapy administration pre guidelines implementation was unsatisfactory level which improved post guidelines implementation, there was high significant statistical positive correlation between nurses’ knowledge and their practice regarding safe and effective administration of supplemental oxygen therapy at pre and post implementation of nursing guidelines. Conclusion: Nursing guidelines effectively improved nurses' knowledge and practices regarding safe and effective administration of supplemental oxygen therapy. Recommendation: Ongoing educational and training guidelines for nurses are needed regarding safe and effective administration of supplemental oxygen therapy and apply the guidelines on large sample selected from intensive care unit at Benha University Hospital.