This study is researching an experimental drug called odronextamab, referred to as study drug. The study is focused on patients with previously treated aggressive B-cell non-Hodgkin lymphoma whose cancer has stopped responding to treatment (also known as 'refractory') or has returned (also known as 'relapsed'). The aim of the study is to see how safe, tolerable and effective the study drug is when given alone. The study is looking at several other research questions, including: * What side effects may happen from taking the study drug versus Standard of Care (SOC) * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drug (which could make the study drug less effective or could lead to side effects) * Comparing the impact from the study drug versus SOC on quality-of-life and ability to complete routine daily activities