You probably want to set up an AWS IAM user that has permission to your AWS Cognito. Then your AWS IAM user can generate an access key to use in n8n.

https://docs.n8n.io/integrations/builtin/credentials/aws/#using-api-access-key

OAuth is more for authenticating multiple other users. For example, say you had many users and they each had different permissions. In this case, you might need OAuth so that the user HAS to sign in and you can only do what they have permissions to do.