By 1920, he left to spend some time in Prague, returning in 1922 to continue his studies at the newly founded Academy of Fine Arts.
The group focused on natural subjects, and promoted the development of a strong native style, rather than simply copying techniques from elsewhere in Europe.
Other members of the group were sculptors Antun Augustinčić and Frano Kršinić; painters Vinko Grdan, Krsto Hegedušić, Leo Junek, Omer Mujadžić, Kamilo Ružička and Ivan Tabaković; and the architect Drago Ibler.
The lyrical expressionism movement was spreading across Europe, and in Croatia many artists such as Edo Murtic were developing natural themes in a very individual way.
[2] Postružnik himself focused on what might be called micro-landscapes,[2] such as his series of individual leaves painted large-scale, and interpreted in shimmering colours.
In addition, his paintings have appeared in special themed exhibits of major Croatian artworks of the twentieth century.
[3][4] During his lifetime, Oton Postružnik held many solo exhibitions in addition to participating with the Earth Group.