Context: In addition to opioids, oxygen therapy is often administered to cancer patients suffering from dyspnea before death, but there are few reports of adverse effects of this treatment. Objectives: To clarify the frequency of adverse effects of oxygen therapy for hospitalized cancer patients before death. Methods: A retrospective study of all patients who died and were discharged from the palliative care unit in Komaki City Hospital from January 2016 to June 2018 was performed. Patients to whom oxygen therapy had already been administered at the time of admission were excluded in the study. Results: The study included 257 patients, with 195 (76%) in the oxygen therapy group (O2+). The cumulative rate of oxygen therapy was 36% seven days before death, 54% three days before death, and 76% a day before death. The adverse effects included a sense of restraint for 64 cases (31%), exacerbation of delirium for 27 cases (14%), nasal/oral hemorrhage for 25 cases (13%), dryness for 5 cases (3%), and deterioration of subjective symptoms for 4 cases (2%). Along with these effects, temporary interruption of oxygen therapy was observed in 76 cases (39%). Conclusion: Oxygen therapy was started in half of the cases within 3 days before death. The adverse events to occur frequently were feeling of restriction and exacerbation of delirium.