I have configured a custom SAML app in Google Workspace and configured AWS Opensearch to use SP-initiated SSO.
I am able to access OpenSearch via the URL but I get the following error
Missing Role No roles available for this user, please contact your system administrator.
I guess there are some SAML attribute mapping and group set up that are required. I've created an opensearch group in Google for users that I wish to have access.
I think the following AWS Opensearch parameters require a value
SAML master backend role Subject key Roles key
This was resolved using and IdP initiated authentication flow instead. The links posted in answers above helped us implement this.