From an exam perspective, they're both the same, and both leverage ElasticSearch, which was renamed in one for OpenSearch.
If in the exam, you get the wording "OpenSearch", please let me know and I will update the course content
Read more here if you're curious: https://aws.amazon.com/blogs/aws/amazon-elasticsearch-service-is-now-amazon-opensearch-service-and-supports-opensearch-10/