Online harassment is defined as repeated malicious internet behaviors intended to upset or disturb victims, often conveying explicit or implicit threats. While anyone can be a victim, there is ample evidence that younger and marginalized people experience online harassment more frequently and of greater severity; designing interventions is a critical tool in supporting justice, equity, diversity, and digital inclusion. This study examines strategies to reduce online harm by promoting bystander intervention. Using the Truman Platform, we can manipulate intervention design affordances of a social media platform and observe the impact on user intervention behaviors and attitudes. The study further examines how personality traits may play a role in moderating bystander actions. We recruited participants via Prolific from February to April 2024, with 137 individuals completing the experiment. The findings will inform better design to support and encourage bystanders to intervene.