OOC:
Not sure if it's the case in OSRIC, but in ADnD 1ed, your "to hit" bonuses couldn't make a natural roll of 19 or less go above 20. Only with a natural twenty and a bonus could a character reach a to hit roll above 20. That is the reason for the 20s repeated six times on the combat matrices.
So if your burly lvl 1 fighter warrior with a total of +4 due to high strength and specialization rolls an 18, his adjusted roll is capped at 20, and he can hit any target with AC -5 or worse. A peasant (0 HD) that is charging with a spear (+2 to hit) and rolls a nat 20, gets a 22 as a result, and could hit up to AC-6.
The whole point of this rule — which I suppose harkens to the game's wargaming past — was to say that extremely powerful creatures (as expressed in an unearthly AC) couldn't be hit by ordinary folk at all : a demon lord with -8 AC wouldn't be hit even if 300 archers shot at it, even if statistically, there'd be 15 natural 20s in the mix... and that's without considering immunity to non-magical weapons and such, which also made some relatively mid-level monsters impervious to mundane attacks, regardless of the rolls...