Osojnik was born on 29 June 1970 in Kranj. From 1990 to 1995 she studied painting at the Ljubljana Academy of Fine Arts. In 1995 she graduated under Prof Emerik Bernard and continued her postgraduate studies under Profs Metka Krašovec and Lisa Adams, a guest lecturer from Los Angeles. In 1997 she finished her postgraduate studies of painting in the class of Prof Gustav Gnamuš. In the same year she received a Fulbright Scholarship for studies in the USA, thanks to which she spent a year studying at Claremont Graduate University in Los Angeles. She began exhibiting her work in 1991. She held her first exhibition in 1995 in Kranj. She is member of the Association of Slovene Visual Artists Societies (since 2000). She is a painter, illustrator and cartoonist.
She has published three artistic picture books (To je Ernest, 1997; Hiša, ki bi rada imela sonce, 2001, and Polž Vladimir gre na štop, 2003). Her illustrations have been published in the Nedelo weekly (2000/01), whereas her cartoon Jožek has been published in the PIL Plus magazine for teenagers. She has also received several awards for her illustrations, including the Hinko Smrekar Award at the 3rd and 4th Slovene Biennial of Illustration (1997, 1999), the Austrian national award "Ősterreichische Kind und Jugendbuch-Illustrationspreis" for her picture book To je Ernest in 1998 and the Levstik Award in 2003.
At the beginning of her painting career she painted on canvas; later, she replaced the canvas with Plexiglas. She has developed an original layering technique (a combination of fluorescent colours, Plexiglas, artificial resins, light, everyday objects and other materials). Consequently, her paintings are described in many different ways: "membrane paintings" (Brejc, 1997), ready-made paintings, paintings/objects, paintings/installations, paintings/lights.