A tag is a very low-effort way to add meaning to a cost line. Without them, a bill tells you what was used. With them, it tells you who used it and why.
Start with a small set
Resist the urge to define a dozen tags. A short list that people actually apply beats a long one they ignore. A good starting point:
Environment: production, development, test and so on.
Owner: the person or team to ask about this resource.
Project or application: what it is part of.
Cost centre: where the cost should be charged, if finance needs it.
Use defined tags, and make them cost-tracking
On OCI, free-form tags are easy but inconsistent, because people spell values differently. Defined tags live in a namespace with controlled keys and, optionally, allowed values. To appear in cost reports, a tag key needs to be marked for cost tracking.
Use tag defaults
OCI can apply default tag values to everything created in a compartment. This gets you coverage on new resources without relying on anyone's memory. Be aware that defaults apply to resources created afterwards. They are not applied retroactively to resources that already exist.
Measure coverage
Track how much of your spend carries the tags you care about, and how much does not. The untagged share is the number that tells you whether your reports can be trusted, and it should fall over time.
Keep it alive
Review tags when you review spend. If a value is wrong, fix it at the source. If a tag nobody uses is still in the list, remove it.