Link, or picture please, don't you see the recessed area in the cup? Standard 2.5 mm plugs are flat on the end, the Oppo plug shows a stepped notch! Have you tried using a standard 2.5 mm plug on a Sennheiser HD700, doesn't work their either.
Like I said look closely at the video that Mike posted, it will show you the recessed area in the cup, and the notch on the plug!
Give me a picture or a link, I have lots of cables I'd like to try on the PM-1, if I had that plug in my hands!
I'm simply saying that if aftermarket companies wanted to make a cable, they could. I don't see any reason a company couldn't copy the shape of that, and since 2.5mm mono connectors are nothing special, make aftermarket cables.
Now what you own now may not work, but that's not to say it can't happen for non OEM cables.
Why recess the male connector? Only Oppo can comment on that. My guess would be to protect the male connector, since if it terminated flat there, without the notch, it would be easy to bend the little 2.5mm male part.
But that's just a guess.
Thanks for the link, that looks exactly like the plug they use!