One thing to watch out for is that you pick your "home region" when you create your account. This cannot be changed later, and your "Always Free" instances can only be created in your home region (the non-free tier doesn't have that restriction).
So choose your home region carefully. Also, note that some regions have multiple availability domains (OCI-speak for availability zones) but some only have one AD. Though if you're only running one free instance then ADs don't really matter.
A bit of a nitpick. You get monthly credit for 4c/24gb on ARM, no matter the region. So even if you chose your home region poorly, you can run those instances in any region and only be on the hook for the disk cost. I found this all out the hard way, so I'm paying $2/month to oracle for my disks.
I don't know the details but I know I made this mistake and I still have my Free Tier instances hosted in a different region then my home. It's charged me a month of $1 already so I'm pretty sure it's working.
So choose your home region carefully. Also, note that some regions have multiple availability domains (OCI-speak for availability zones) but some only have one AD. Though if you're only running one free instance then ADs don't really matter.