Old Norwegian (Norwegian: gammelnorsk, gam(m)alnorsk) is an early form of the Norwegian language that was spoken between the 11th and 14th century; it is a transitional stage between Old West Norse and Middle Norwegian. Its distinction from Old West Norse is a matter of convention.
One of the most important early differences between Old Norwegian and Old Icelandic is that h in the consonant combinations hl-, hn- and hr- was lost in Old Norwegian around the 11th century, while being preserved in Old Icelandic. Thus, one has e.g. Old Icelandic hlíð 'slope', hníga 'curtsey' and hringr 'ring' vs Old Norwegian líð, níga and ringr, respectively. Old Norwegian had an alternative dual first person pronoun, mit, to the Common Norse vit.
The plagues that decimated Europe in the Middle Ages came to Norway in 1349 (Black Plague), killing over 60% of the population. This probably precipitated the current process of language development. The language in Norway after 1350 up to about 1550 is generally referred to as Middle Norwegian. The language went through several changes: the grammar was simplified, including the abandonment of grammatical cases and the removal of personal inflection of verbs. A vowel reduction also took place, in some dialects, including in parts of Norway, reducing many of the last vowels in a word to a common "e".
The phonemic repertoire also underwent changes. The dental fricatives, represented by the letters þ and ð disappeared from the Norwegian language generally merging with their equivalent plosive sounds, represented by t and d respectively.
«Dokumentasjonsprosjektet» Includes old Norse dictionary and includes Diplomatarium Norvegicum which is a collection of texts from before 1570 (originally 22 books with 19 000 documents) and Regesta Norvegica which contains letters and official documents (it's a more detailed description on "Om Regesta Norvegica") from 822 to 1390. This page is partly available in English. (not Regesta Norvegica)