OPNsense is an open source, FreeBSD-based firewall and routing software developed by Deciso, a company in the Netherlands that makes hardware and sells support packages for OPNsense.

[3] When m0n0wall closed down in February 2015 its creator, Manuel Kasper, referred its developer community to OPNsense.

[5] Along with acting as a firewall, it has traffic shaping, load balancing, captive portal and virtual private network capabilities, and others can be added via plugins.

[6][7] The software also offers next-generation firewall capabilities utilizing Zenarmor, a NGFW plugin developed by OPNsense partner[8] Sunny Valley Networks.

[9] In November 2017, a World Intellectual Property Organization panel found that Netgate, the copyright holder of pfSense, used the domain opnsense.com in bad faith to discredit OPNsense, and obligated Netgate to transfer domain ownership to Deciso.