I've always understood the difference roughly to be that f/opsec is around keeping safe from counterinsurgency as people engaging in anti-authoritarian action where f/privacy is more about measures to keep safe from everyday surveillance. So different focuses on a similar concern.

However, there are even more forums, f/netsec and f/security, that are probably the result of people confusing things, in part because people don't know to look for f/opsec.