Background: Rule OP-35, which characterizes treatment-related complications of patients receiving outpatient chemotherapy that result in a potentially avoidable emergency department (ED) visit or hospitalization, was developed to encourage practices to build treatment models that reduce such events. However, defining visits as potentially avoidable based on symptoms may not capture the complexity of caring for oncology patients. We aim to evaluate the effectiveness of OP-35 in identifying preventable ED visits by real world standards at an academic institution. Methods: A retrospective analysis was performed reviewing ED visits at the Sidney Kimmel Cancer Center (SKCC) at Thomas Jefferson University for oncology patients from 10/1/2020 to 1/31/2021. Each patient received care at SKCC had received intravenous or oral chemotherapy in the preceding 30 days, and each encounter was classified as potentially avoidable by OP-35 criteria. Two investigators independently conducted chart reviews to determine whether these visits were potentially avoidable, recording whether the patient attempted to contact their care team prior to the ED encounter and assessing if the concern could have been managed in a timely manner in an outpatient setting. The two records were then compared, and the principal investigator served as an arbiter for determining if a visit was potentially avoidable in instances where the investigators disagreed. Results: We reviewed 144 total encounters and excluded events from patients with either acute leukemia or breast cancer on hormone therapy only, leaving 107 encounters for analysis. After evaluating the clinical circumstances, we determined that 29% of these ED encounters were potentially avoidable. Applying New York University Emergency Department Algorithm (NYU-EDA) criteria, 69% of encounters were considered potentially avoidable. Patients called for advice before seeking ED care in 53% of unavoidable encounters compared to 26% of potentially avoidable encounters. An additional 14% of visits deemed unavoidable were from patients sent directly from clinic. For potentially avoidable encounters, 60% of patients were discharged directly from the ED. In comparison, 8% of unavoidable encounters led to discharge from the ED. Pain was the most common reason for encounters and 53% of these visits were considered potentially avoidable. Conclusions: We found that approximately 30% of ED encounters deemed avoidable by OP-35 criteria were considered potentially avoidable following clinician review. In the majority of cases patients were referred to the ED following initial outpatient attempts at management. NYU-EDA criteria for preventability did not correlate with OP-35 nor clinician consensus regarding potentially avoidable encounters. More work remains in refining tools to identify potentially avoidable ED visits for oncology patients.