Reference: https://www.oracle.com/a/ocom/docs/cloud/block-volume-100.pdf#page=10

Budgets Overview: A budget can be used to set soft limits on your Oracle Cloud Infrastructure spending.
You can set alerts on your budget to let you know when you might exceed your budget, and you can view all of your budgets and spending from one single place in the Oracle Cloud Infrastructure console.
How Budgets Work: Budgets are set on cost-tracking tags or on compartments (including the root compartment) to track all spending in that cost-tracking tag or for that compartment and its children. All budgets alerts are evaluated every hour in most regions, and every four hours in IAD. To see the last time a budget was evaluated, open the details for a budget. You will see fields that show the current spend, the forecast and the "Spent in period" field which shows you the time period over which the budget was evaluated. When a budget alert fires, the email recipients configured in the budget alert receive an email. Budget Concepts: The following concepts are essential to working with budgets:
BUDGETA monthly threshold you define for your Oracle Cloud Infrastructure spending. Budgets are set on cost-tracking tags or compartments and track all spending in the cost-tracking tag or compartment and any child compartments. Note The budget tracks spending in the specified target compartment, but you need to have permissions to manage budgets in the root compartment of the tenancy to create and use budgets.ALERTYou can define email alerts that get sent out for your budget. You can send a customized email message body with these alerts. Alerts are evaluated every hour in most regions (every four hours in IAD), and can be triggered when your actual or your forecasted spending hits either a percentage of your budget or a specified set amount. Required IAM Policy Reference:
https://docs.cloud.oracle.com/en-us/iaas/Content/Billing/Concepts/budgetsoverview.htm