Ossec is described as 'OSSEC is an Open Source Host-based Intrusion Detection System. It performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response'. There are three alternatives to Ossec for Linux, Windows, Mac and BSD. The best alternative is Tripwire, which is both free and Open Source. Other great apps like Ossec are Osquery (Free, Open Source) and Verisys (Paid).
osquery is an operating system instrumentation framework for Windows, OS X (macOS), Linux, and FreeBSD. The tools make low-level operating system analytics and monitoring both performant and intuitive.
The Verisys File Integrity Monitoring system is a powerful software solution for Windows and Linux systems, with cross-industry applications including PCI and SOX compliance and data integrity assurance.