Apache Cassandra and OpenSearch are two popular options in the vector database space. Both technologies have evolved to include vector search capabilities as an add-on, making them suitable for AI-driven applications. Key differences between the two include their search methodology, data handling, scalability and performance, flexibility and customization, integration and ecosystem, ease of use, cost considerations, and security features. The choice between Cassandra and OpenSearch should depend on specific use cases, data types, scalability needs, and existing technology stacks.