My RM2 books are hardcopy and at home, but I could swear it's in there somewhere.
Checking the RMC PDFs it's RMC AL pg 19-20.
I've rarely seen a 0 OB attack do much, but fumbles on full parry can drasticly affect the fight.
OTOH, I don't allow things like weapon bonus OB to be used for parry, OR spells that give an OB/DB bonus like some on paladin's "Arms Way" I don't allow conversion of that OB to DB either. Another Paladin list "Inspirations" gives a bonus to a "Die Roll" which is also OB, can't be DB. . .
So Hypothetically, you could have a row of imperial elite riot supression troops armed with +10 quarterstaves backed up by a high level Paladin facing off with a horde of rioting citizens. . . .The cits charge, the paladin casts Inspirations X. . . . .
The Imperials use their 100 OB of skill for "Full parry". . . .So +100 DB. . .but still have +10 weapons and a +50 "To their die rolls" spell effect on them. . .or +60 OB . . .which against a mob would likely be enough to break the crowd's heads. . .
Now, that's a bit of a rarified example, but I can think of plenty of situations where "Full Parry" ended up leaving a decent amount of "Forced OB" behind.
I wonder, is that a house rule? Could you add the +10 Magic Weapon bonus of the staves to Parry? (The "Inspirations" would seem to have to be OB, since it affects die rolls.)