A lot of the issues people have with Oracle is caused by using not using a normal credit card or not keeping a valid card on file. A bunch of failed signup attempts will probably get your account flagged. When you sign up for free tier, you will be charged a couple of times, but it will be returned within a few days. You can't do anything on free tier that will cost you money. You would have to upgrade to pay as you go, which requires re entering your credit card info, if you want to use anything that's not free.

Don't run any public proxies or VPNs. Your account will be gone as soon as they get a DCMA notice. If they get complaints about anything you are hosting, they will likely terminate your account as well. Don't do anything even remotely related to crypto currency. Don't do a bunch of port scans or ping a large nunber of IPs. Don't stop or remove the Oracle services that come with their images either.

If your instances don't have enough load on them, they will be stopped after about a week. You can always restart them, but that can be difficult with the ARM instances since they are in high demand.