Average CPU utilization oracle database
Start using your licenses in a smart way. Many customers buy EE licenses, only using SE features. Don't underestimate the power of SE. You can use SE with a max of 4 sockets and with modern CPU's that's a lot of power. SE1 has a max of 2 sockets. See Database Licensing - Oracle

Next to that, consider consolidating your smaller applications into 1 serious database, often leading to much better CPU and license utilization. For one customer I saved 800.000,-/year doing so.

The question about how to size for over provisioning is really a business question. How does the business get hurt when it can't process every request immediately? Some will lose a lot of money for lost sales, others can handle it.