Offspring of a/o shapes

There are some words in the English tung which in the bygone were written with either a or o. It is often said that o shapes stem from the south, whereas a shapes stem from the north. While this calls for a more in-depth look, the following is only a short rundown of what became of a and o shapes later. May it be of some help to settle on the nowaday shape of a word of old when it is brought back.

-n
Hark that on, swan, and wan share the same loud in many a bytung of English, but only one is written with an o.

-ng
For there is no sheer overweight of words with either a or o shapes, it is best to look at each word to be brought back on its own. Headwords of wordbooks can give some hint as to how to write them.