Advances in medical care for oncology patients have heightened the complexity of this patient population, leading to an increased need for lifesaving, critical care. Yet, there is a lack of literature on education to prepare nurses to provide care to critically ill oncology patients. The purpose of this quality improvement project was to evaluate oncology nurses' mastery of basic and oncology-specific critical care knowledge after an evidence-based supplemental training intervention. An 8-hour supplemental training intervention was developed to increase oncology nurses' knowledge about basic and oncology critical care by addressing score deficiencies noted on the Basic Knowledge Assessment Tool (9r version). Seventeen oncology nurses completed the supplemental training intervention. The Basic Knowledge Assessment Tool and an Oncology Knowledge Survey were administered pre and post supplemental training, and significant changes were detected with Wilcoxon signed rank tests. Mean total Basic Knowledge Assessment Tool scores significantly improved from 65.7 before initial course to 73.7 post supplemental training (P = .002). Oncology Knowledge Survey total scores increased from 75.3% to 80.9%, which was a statistically significant improvement (P = .039). Critical care training is imperative for oncology nurses preparing to care for high-acuity oncology patients. Adequate knowledge of basic and oncology-specific critical care is important to prevent adverse events and improve outcomes for this complex patient population.