Natural 20's in My OD&D campaigns

Over the years I have seen many, many discussions on message boards about "does a natural 20 always hit." I adjudicate it as follows: If magic is not required to hit and you roll a natural 20, then you always hit. If magic is required and you are using a magic weapon (with or without a plus to hit) and you roll a natural 20, then you always hit. Those are the simple ones.

Let's say that you roll any number on the d20 from 2-19 (a natural 1 always misses) and your roll plus any bonuses you have equals or exceeds the number needed to hit, then you always hit (again accounting for whether or not magic is required or not and whether magic is used or not).  Again still simple.

Now for example, let us say that you need a 20 to hit AC 0 and the AC you are trying to hit is AC -2, what do you need to roll to hit (again accounting for magic as per the above)? Well there are two ways you can hit AC -2, the first is to roll a natural 20 and you hit, the second is to roll any number and consider any bonuses you have to hit. If you roll a 18 and have a +4 to hit, then you would hit AC -2 by rolling a 18. And so on and so forth, that is the way I run it and have never run it any different way. That is one of my very oldest house rules.