head to head · open source
opa vs ossec-hids
opa has 12,256 GitHub stars, 1,686 forks, 310 open issues and last shipped yesterday. ossec-hids has 5,057 stars, 1,074 forks, 125 open issues and last shipped 3 days ago. opa leads on adoption by 142% (12,256 vs 5,057 stars). opa is written in Go under Apache-2.0; ossec-hids is written in C under GPL-2.0. opa has attracted 14% as many forks as stars, ossec-hids 21%. opa was the more recently maintained of the two, and both are self-hostable with no licence fee. The two share 1 topic tag (compliance), so they are genuine substitutes rather than adjacent tools.
Two open source projects, one decision. Both are free and self-hostable — the differences are community size, license terms, language stack and release pace.
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Side by side
| opa | ossec-hids | |
|---|---|---|
| GitHub stars | ★ 12K | ★ 5.1K |
| License | Apache-2.0 | GPL-2.0 |
| Written in | Go | C |
| Last push | 2026-09-19 | 2026-09-17 |
| Forks | ⑂ 1.7K | ⑂ 1.1K |
| Self-hosting | Yes | Yes |
| Data ownership | Your server | Your server |
pick opa if
- You weight community size — 12K stars and counting
- You want the Apache-2.0 license terms
- Your stack matches Go
- You value the larger contributor base for long-term maintenance
pick ossec-hids if
- You want the ossec-hids feature set and don't need the biggest community
- You prefer the GPL-2.0 license terms
- Your stack matches C
- You evaluated both and ossec-hids fits your workflow better
About opa
Open Policy Agent (OPA) is an open source, general purpose policy engine that enables unified, context aware policy enforcement across the entire stack. It lives in the cloud native ecosystem as a graduated project in the Cloud Native Computing Foundation landscape, and it is written in Go under the Apache 2.0 license. Policy is expressed declaratively in the Rego language, and the project ships alongside a CLI, an HTTP REST API, a Go SDK, and editor tooling rather than as a single binary alone. The repository has been active for roughly eleven years, with 12,234 stars, 1,679 forks, and a last push dated 15 Septe…
About ossec-hids
OSSEC is an open source host based intrusion detection system written in C and released under the GNU General Public License version 2. It combines several security functions into one platform: log analysis, file integrity checking, policy monitoring, rootkit detection, real time alerting, and active response. The project describes itself as a full platform to monitor and control systems, mixing the aspects of HIDS, log monitoring, and SIM/SIEM into a single solution. It lives in the security and compliance tooling ecosystem, with topics spanning intrusion detection, file integrity management, log analysis, PCI D…
read the full ossec-hids overview →
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Frequently asked questions
Is opa or ossec-hids more popular?
opa has 12,256 GitHub stars and ossec-hids has 5,057. opa has the larger community by that measure.
Are opa and ossec-hids free?
Both are open source. opa is licensed under Apache-2.0 and ossec-hids under GPL-2.0. Neither carries a licence fee.
What is the difference between opa and ossec-hids?
opa is written in Go and ossec-hids in C. The practical differences are community size, licence terms, language stack and release cadence — all compared in the table above.
Which should I choose, opa or ossec-hids?
Choose opa if you want the larger community (12,256 stars) or its Apache-2.0 licence terms. Choose ossec-hids if its feature set, stack or GPL-2.0 licence fits better. Both are self-hostable.