This is a free gem that seems to be pretty unknown outside of chip tune circles. Apparently it is not finished yet but it may be abandoned at this time. Still, the sound is excellent and it is definitely worth a try even if you have other FM VST's already.
The controls on this synth are not intuitive to figure out due to the slightly different naming conventions and abreviations and the lack of an English manual. There seems to have been an English translation at some point but the links are dead now. After watching one of the youtube tutorials on this synth everything makes sense though and it is a fairly simple synth to operate as FM synths go. The sound is excellent and really nails the 4 operator Yamaha sound. I did a comparison with a simple bass patch from an old Yamaha PSR keyboard. I tried to recreate it in VOPM, Dexed, and FM8. Surprisingly, I could only match it with VOPM no matter how long I playing with envelopes and levels. VOPM also seems to recreate by default some of the digital noise artifacts of the OPM chip. I'm not sure if these are due to aliasing or truncation due to the low resolution envelope and sine wave table lookup math. Regardless of the cause, VOPM has a grungy slightly low fi sound depending on the patch.
This library will not work on the Mac version because, while these are technically text files, they are Windows text files, and the Mac version of this VST only seems to understand Unix text files. The difference is DOS and Windows text files end with both the line feed and carriage return ASCII characters, where Unix text files end with just the line feed.
There are some methods of converting between the two file formats, but with more than 2600 files in this archive, anyone undertaking a conversion is in for a long slog.