If you want the closest thing to having the actual OB-X keyboard this is the plugin to get. I bought this in 2015 and it is still by far the best plugin for this style of keyboard I have yet to use. I own this and there are other good ones out there (that I also own) but if you want the best, this is it.
OK I might be a little bit subjective here, let me explain :
I realised quite recently that the OB-X synth (the real machine, not the VST) had the sound I've been looking for in the last 15 years. No kidding. Since I've started making music as a kid, I wanted THAT sound but I had no idea where to find it.
Of course buying an OB-X wasn't an option so I looked for a similar VST. That's when I found this little gem !
I've tried MANY a VST synth in my life : all the native instruments, alchemy, sylenth-1, minimogue, ableton's operator and a lot of freewares. I really found them good, until I listened to that one.
They claim it has an analog sound which I found hard to believe given that everybody pretends to sound analog nowadays. To me it's the only one that sounds analog (the free minimogue is a close match though, but it's a different kind of sound, so they complete each other pretty well). Honestly this one has this "thing" which makes it sound like a real machine.
So yeah, I'm a bit subjective because I discover the OB-X (the real synth) and OP-X (this plugin) at the same time, and got really excited, however I can say to things : - it's defintely a faithful emulation, and if you can hear the difference withthe real thing, then you definitely have superior ears than mine - it's the most analog sounding VST I ever heard
Another thing to consider also is that I'm not into tweaking or designing sounds. I'm more into writing music. So to me presets are just as important as the capabilities of the instrument. That's another area where OP-X shines ! There are 1500 presets, and most of them sound just excelent. You have also a lot of presets from famous songs and artists (pink floyd, toto, etc ...) which are surprisingly similar if not identical.
So in a word, this VST sounds amazing right out of the box.
The only thing is that the interface looks really cluttered and that's a bit annoying. Besides, you will probably have to read the manual, if you want to make the most of it (adding efects, controling the detuning of the voices and so on ...)
Yet it does have the best sound I ever heard and to me that's the most important thing.
If you are after this kind of sound, try the demo. If you aren't ...well try it too, you might fall in love ;)
I bought this VST several months ago because I needed that fat Oberheim sound for some songs my band does live.
This is a good Oberheim incarnation and has all the bells and whistles that came on the original hardware version, plus a few extra. The sound is big and fat just like the Obies of the day. I can do a fairly direct comparison because I still own a Matrix 1000 and have it in my studio so I can hear the differences. The differences are there, but there not so much that this synth can't be considered a fairly faithful recreation from the sound standpoint.
The added capabilities of being able to pan and tune each oscillator individually is really nice. Where this VST lacks though is in the area of Midi implementation and EFX.
Virtually every other "decent" VST out there usually has a built-in chorus, delay or reverb somewhere. For me this is very helpful since I can store the appropriate effect with each patch. The OPX-PRO does not have any onboard EFX. While it sounds big, all the older analog synths benefited from EFX added to the patch and the Obie is no exception. I have to chain in outboard VT EFX to get the sound I need from the Obie and have to program separate patches on those EFX so that they load when I change the patch on the OPX. At $149.00 it could have it's own onboard EFX I think.
Also, no midi learn capabilities so you have to handle controller mapping in the host or on the controller board itself. If you're fine with that then this isn't a huge deal and that is in fact how I'm dealing with it myself. I've just gotten spoiled over the years with the midi learn capabilities of a lot of other commercial VSTs.
In the end, this isn't a bad VST and it sounds great and pretty darn authentic. I would highly recommend downloading the demo version and playing with it a bit to see if it suits your needs.