Which versions of Opus and Windows are you using?
Make sure Opus hasn't been set to run in compatibility mode for an older version of windows, as that can mess up the Explorer Replacement settings so double-clicks within Explorer don't stay in Explorer:
(If any compatibility option was on, turn it off, then reboot. In Opus, turn off Explorer Replacement and click Apply, wait a few seconds, then turn it back on. That should re-write the registry values with the appropriate ones for your Windows version.)