Edit: Replaced open source product with open source software
Netlify is one example.
You'll want to look into the particular projects license to make sure it is ok.
Our SaaS is built / runs on a phenomenal amount of OSS. The core is not OSS yet, we create some OSS for non differentiating code, contribute back to the web framework we use.
The exceptions arise when the business needs a particular service that they don't have the personnel or expertise to develop and support in-house, especially when that service is customer facing and is required to meet their service level agreements. Even then, that service may be comprised of open source and close sourced software, but commercially supported by a 3rd party.