NH-U12S Max TDP on LGA1151 socket: 95W & OC**
i5-8600K TDP: 95W
So yes, it will be cool enough without OC, if you want to OC Noctua says this:
**Our recommendations assume that the cooler is installed in a typically equipped, properly ventilated system. If you are using particularly hot components inside your system, it may be necessary to use stronger case ventilation to ensure sufficient CPU cooling performance.