Download PuTTYGen.exe from the PuTTY site, load the key into it, then save it out again in PuTTY's format. That will then work with Opus (and other things based on PuTTY/Pageant).
Download PuTTYGen.exe from the PuTTY site, load the key into it, then save it out again in PuTTY's format. That will then work with Opus (and other things based on PuTTY/Pageant).