Rather than wiping away the lube deposited by the bullet that was just fired, perhaps the OP card wipes away the fouling that was deposited in the bore from the previous shot?
If it can do one, then it can also do the other.
I don't know what purpose the old ammunition manufacturers intended for the OP card in their ammo.
There's a chance that the OP card increases velocity, and it could also affect accuracy.
Some experimentation and assessment may be required to determine whether or not the OP card actually serves a useful purpose in your loads.
If both types of loads could be chronographed with and without OP cards then that would be even better.