Relation with other West Germanic languages
In the Middle Ages, a dialect continuum existed between Old Dutch and Old Saxon, a continuum which has only recently been interrupted by the simultaneous dissemination of standard languages within each nation and the dissolution of folk dialects. Although they share some features, a number of differences separate Old Saxon, Old English, and Old Dutch. One such difference is the Old Dutch utilization of -a as its plural a-stem noun ending, while Old Saxon and Old English employ -as or -os. However, it seems that Middle Dutch took the Old Saxon a-stem ending from some Middle Low German dialects, as modern Dutch includes the plural ending -s added to certain words. Another difference is the so-called "unified plural": Old Saxon, like Old Frisian and Old English, has one verb form for all three persons in the plural, whereas Old Dutch retained three distinct forms (reduced to two in Middle Dutch).
Old Saxon (or Old Low German) probably evolved primarily from Ingvaeonic dialects in the West Germanic branch of Proto-Germanic in the 5th century. However, Old Saxon, even considered as an Ingvaeonic language, is not a pure Ingvaeonic dialect like Old Frisian and Old English, the latter two sharing some other Ingvaeonic characteristics, which Old Saxon lacked. This, in addition to the large number of West-Germanic features that Old Saxon displayed, had led some philologists to mistakenly think that Old Dutch and Old Saxon were variations of the same language, and that Old Saxon was an Istvaeonic language.
Relation to Middle Low German
Old Saxon naturally evolved into Middle Low German over the course of the 11th and 12th century, with a great shift from Latin to Low German writing happening around 1150, so that the development of the language can be traced from that period.
The most striking difference between Middle Low German and Old Saxon is in a feature of speech known as vowel reduction, which took place in most other West Germanic languages and some Scandinavian dialects such as Danish, reducing all unstressed vowels to schwa. Thus, such Old Saxon words like gisprekan (spoken) or dagō (days' – gen. pl.) became gesprēken and dāge.