Purpose: This study aimed to explore the practices of oncology nurses in the management of chemotherapy-related oral mucositis (OM) by MASCC/ISOO guidelines. Methods: This descriptive and cross-sectional study was conducted between December 25, 2021, and January 31, 2022, with 157 oncology nurses in Turkey. Data were collected through OM Practices Assessment Form. Results: The mean age of the nurses was 29.05 ± 7.40, the majority (90.4%) of them were female and 76.4% of them have a Bachelor’s degree. More of the nurses (59.9%) had a written protocol for managing OM in their institutions, 38.9% of them used the MASCC/ISOO guideline, and 63.0% of them used an OM assessment scale. Most of the nurses (99.4%) recommended mouthwash to patients and 65.6% of them recommended mouthwash four times and more a day. 54.1% of the nurses recommended saline (%10.8) or carbonate (%36.9) or a mixture of saline and carbonate (%6.4) solutions for mouthwash. Despite the lack of sufficient evidence in the MASCC/ISOO guidelines regarding black mulberry syrup, 45.2% of the nurses recommended it for the prevention of OM and 43.3% of them suggested the treatment of mucositis. It was found that 82.0% of nurses who followed MASCC/ISOO guidelines recommended to patients implement oral care four times and more a day, while 55.2% of them who did not follow MASCC/ISOO guidelines recommended four times and more a day. The difference was found to be statistically significant (χ2 = 11.836; p = 0.01). Conclusion: It was determined that there were deficiencies in the implementation of written protocols for OM and the use of guidelines, and there were differences between the frequency of oral care, oral care products, and the practices of nurses in the prevention and treatment of OM.