The other thing to note is that the OSP comes in several sizes and handle types. It is a handmade blade afterall which is its big selling point.
I have the Virtuoso and the Primorac. I would not say that the OSP is OFF- as it is slower than my Viscaria which I think is the 'Forum' standard for an OFF- blade. Then again, mine is a 158mm blade so the relative slowness may be due to its bigger size.
Both blades are very popular due to the consistency of good quality in their manufacture. Neither has any advantage over the other and any performance difference will have minor effect on your overall game result.
So choice is down to the usual price and ergonomics. For me, the OSP won because I could choose the size as I was in the mood for a bigger than normal (156 or157mm) blade.
BTW, if you look up Daniel Moses in the TTDSL videos, he uses the Primorac blade with Tenergy 05.