This is one of my most confusing and disagreeable aspects of RM. I personally think (and often think was the intent of the creators) that your modifiable OB (for OB/DB split, or restricted by lower skill) is your skill bonus, and does not include quality/magic, position, phase, etc bonuses. Lumping all these factors into one "OB" is terribly limiting, and can not reflect changes in the combat environment during the round because you have to declare your split at the beginning. It also creates too many problematic and nonsensical scenarios.
And he wants it to be applied after splitting OB for DB. So, if you have a +30 magical broadsword, with a skill of +70 not including the +30, and you go for a +0 OB putting your +70 to DB, you would still get an attack at +30, the weapon's magical bonus. I see the appeal to that, and I am pretty-sure that the reason the game doesn't do it that way is for the sake of ease. But, I think it is still better to just add it and then do all the calculating. Mostly for ease. It just doesn't matter that much to me, I guess.
If I've got a +30 sword of slaying, and my opponent has a rusty shortsword, but we have equal riding skills, it boggles the mind that we would attack with the same modifier. It's painful enough that the Swordmaster (100 weapon skill) and the knave (30 weapon skill) would have the same capability on horseback if they each had a 30 riding skill.
I just think so many problems are solved when you separate your modifiers from your weapon skill. Its simpler, clearer, more realistic, and is more able to account for changes in modifiers during the round. It separates offensive modifiers from defensive modifiers, and doesn't clump everything in to one immutable and unrealistic number.