Its distinction from Old West Norse is mostly a matter of convention, but it is also the period when the language begun to develop its immense diversity.
[5] The u-umlaut of short /a/ (written ǫ in normalized Old Norse) is not as consistently graphically distinguished from non-umlauted /a/ as in Old Icelandic, especially in writings from the Eastern dialect areas.
[7] Old Norwegian had alternative dual and plural first person pronouns, mit, mér, to the Common Norse vit, vér.
Sources from the 17th and 18th century report that Norn, often misidentified as Danish, Norse or Norwegian, was in a rapid decline, although prevailing in Shetland more than Orkney.
The language went through several changes: morphological paradigms were simplified, including the loss of grammatical cases and the levelling of personal inflection on verbs.