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When you've reached the end of your 30-day trial or used all your Free Trial credits (whichever comes first), youll be notified and will have a grace period of 30 days, starting from the expiration date, to upgrade to paid. You will no longer be able to create new paid resources, but your account will remain active. Your resources will continue to exist for a few days, allowing you to upgrade your account and keep your resources before they're reclaimed by Oracle. If you do not upgrade to paid by the end of the grace period, your Free Trial service instances and data will be deleted. (Note that reclaimed resources cant be recoveredthey are permanently deleted.)

Resources identified as Always Free will not be reclaimed. After your Free Trial expires, you'll continue to be able to use and manage your existing Always Free resources, and can create new Always Free resources according to tenancy limits.

However, if you have more Ampere A1 Compute instances provisioned than are available for an Always Free tenancy, all existing Ampere A1 instances are disabled and then deleted after 30 days, unless you upgrade to a paid account. To continue using your existing Arm-based instances as an Always Free user, before your trial ends, ensure that your total use of OCPUs and memory across all the Ampere A1 Compute instances in your tenancy is within the Always Free limit.

See more here:
Cloud Free Tier | Oracle