Security testing: How does Owasp test assess the vulnerability of a system or a network?

This is rather very unspecified question. I don't think that something like "OWASP test" even exists, as from my experience you are most probably reffering to "OWASP top 10" or "OWASP ASVS", which both are rather set of rules/vulnerabilities around applications and web applications mainly, not "system" or "network". So answear to that one will be: "They don't assess anything beacuse something like OWASP test doeasn't exist".