Database links in RDS Oracle are possible for homogeneous scenarios only. For heterogeneous scenarios you would need access in instance level in order to install drivers and configure Oracle Heterogeneous Services to do this, which is not possible in RDS.
I have already configured HS (to mySQL) in a Oracle running in EC2, if it is really necessary. I don't recommend because it's hard to handle sessions, I would go deeper in the case and tech requirements, looking for alternatives.
To refresh the answer: since end of October 2021, AWS offers an additional deployment option, "RDS Custom for Oracle" https://aws.amazon.com/about-aws/whats-new/2021/10/amazon-rds-custom-oracle/ , with this deployment option the user has access to the underlying host.
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