[5] Until 1990 Orešnik focused on Germanic comparative linguistics and on Scandinavian languages, especially Icelandic.

After 1985 Orešnik, together with a team of younger colleagues, developed a theory of strong and weak variants in syntax within Natural Linguistics; that is, synonymous syntactic units that compete with one another in the history of the language.

The group organized two international conferences on Natural Linguistics (including the theory of strong and weak variants) at the University of Maribor, Slovenia, in 1993 and 1996.

Orešnik married the Slavic scholar and translator Borghild Birgitta Gyllenberg (1931–1980).

[8] Orešnik died on 1 April 2024, at the age of 88,[3][4] and was interred at Ljubljana's Žale Central Cemetery.