He studied there until 1885, and continued as a student under the Danish painters Laurits Tuxen (1853–1927) and Peder Severin Krøyer until 1886.

[2] Since so much of what Osa had studied underlined the continuity between medieval and folk art decorations, it was natural for him to examine Norwegian frescoes.

The most notable example of this was Faderen, which was shown during 1889 at the World Exhibition in Paris (Exposition Universelle).

A memorial of him by the Norwegian-American sculptor Lars Fletre was unveiled in 1967 at the Ulvik National Museum.

Osa wrote the memoir book Med pensel og feleboge in 1954, when he was 94 years old.