“Fixing” an Out-of-Memory error breaks down into three response phases:

• Restarting Solr: As a “first aid” measure, you can restart Solr. For Azure or GCP, a deployment that hangs due to an out-of-memory error will restart automatically. AWS deployments show a message saying “Service Malfunction” and an option to Stop Solr. Once Solr has stopped, the option to Start Solr appears. You can proceed from there.
• Diagnosing Root Cause: See this SearchStax Blog post by Dipsy Kapoor for a deep dive into features that can create OOM exceptions.
• Upgrade Deployment: When you can’t finish loading your data due to repeated Out-of-Memory errors, consider upgrading to a deployment with more memory. This is usually very easy to do. Clients with premium support agreements (Gold, Platinum) may request assistance with an upgrade.