OOM killer terminating database on AWS EC2

We have a database on an AWS EC2 server that the OOM killer has terminated twice in the last 5 days, both times it was the ora_dbw0_dwprod process. On 1/8 postfix was enabled to allow us to email the DBA team through an AWS relay server when a backup failed. We stopped running daily backups and cronjobs that did a quick check for expired accounts. We've left postfix enabled for sending emails. We are searching for answers but have none yet as to why this is happening. We also no longer have Oracle support available to us. (management saving money again).

Questions:

1. Could postfix be related to the memory issues even though we haven't sent any emails since the first crash 5 days ago?
2. How can we monitor the memory usage of an EC2 instance?
3. How do you disable the OOM killer in EC2 should we decide to go that route? (we have it disabled on our on-prem servers) The docs I've found so far have not been helpful.

I appreciate any help you can give us or pointing us in the right direction.