
The water colors and temperas he showed drew favorable comment from Howard Devree, critic for the New York Times, who said his realist and semi-abstract landscapes were vigorous, germane, and expressive and from Jerome Klein of the New York Post, who commended Opper's "sparkling brilliance and unfailing vivacity." Before leaving the Artists' Congress he helped organize its fourth annual exhibition in 1940. Opper was given his first solo exhibition at the Artists Gallery in 1937. By his account, during these two years his work was both semi-abstract and anti-war. He grew disenchanted with this organization, in turn, and left it after submitting work to two of its group exhibitions. Within the year, Opper left the Artists Union and joined the American Artists' Congress. At the same time, he joined the Artists Union and became business manager of its journal, Art Front. He later said that the project was a lifesaver for impoverished artists, particularly abstract artists such as himself. In 1937 the influential critic, Edward Alden Jewell, called this effort a "revolt against literary subject-paintings" and said that the great majority of paintings in a current exhibition were simply "objects." The same year, after a brief attempt to support himself as an art instructor, Opper joined the Federal Art Project in Manhattan as an easel artist and remained for three years. In 1936 Opper became a founding member of American Abstract Artists, a group formed by New York artists to promote and exhibit a style of art that was then derided by critics and shunned by collectors. There he met modernist painters who sought Hofmann's guidance and began to develop his own modernist style. In 1934 Opper moved to Manhattan and a year later began to work in the studio that Hofmann had set up as the School of Fine Arts on East 57th Street.

Hofmann influenced his approach to art, although not as an instructor. There, he met Hans Hofmann, who was teaching at the Thurn School of Art.

Two years later he spent a summer in Gloucester, Massachusetts. He spent the following year in Chicago taking classes at the Art Institute and subsequently returned to Cleveland where he enrolled at Western Reserve University, graduating in 1932.

Graduating about 1926 he briefly studied at the Cleveland School of Art and there encountered the artists Henry Keller, as an instructor, and Clarence Carter, as a fellow student. In his senior year he attended classes at the Cleveland Museum of Art.

While in high school he took art classes and enrolled in a correspondence art course. He became interested in drawing at a young age. Opper was born in Chicago, Illinois, and raised in Cleveland, Ohio. That's what my school of abstract art is about, a school that evolved from nature, not conceptual, not geometric, not hard-edged. He said, "The whole is the sum of its parts. Late in life, he described his style by what it was not. He was associated with the abstract expressionist movement and frequently showed in galleries that specialized in abstract expressionist art. He became known for his handling of color and in particular his ability to create dramatic intensity on the picture plane by means of juxtaposed, more-or-less rectangular areas of color. John Opper (1908–1994) was an American painter who transitioned from semi-abstract paintings in the late 1930s to fully abstract ones in the 1950s.
