In 1894, Oppenheim was the author of a textbook on nervous diseases titled Lehrbuch der Nervenkrankheiten für Ärzte und Studierende, a book that soon became a standard in his profession.

He also published significant works on tabes dorsalis, alcoholism, anterior poliomyelitis, syphilis, multiple sclerosis and traumatic neurosis.

In 1889 he published a treatise on traumatic neuroses that was harshly criticized by eminent physicians that included Jean-Martin Charcot (1825–1893) and Max Nonne (1861–1959); the reason being due to Oppenheim's assertion that psychological trauma caused organic changes that perpetuated psychic neuroses.

He coined the term "dystonia musculorum deformans" for a type of childhood torsion disease he described, a disorder that was later to become known as "Ziehen-Oppenheim syndrome" (named along with German psychiatrist Theodor Ziehen 1862–1950).

His son Hans Oppenheim became an opera conductor and emigrated to Britain following the Nazi takeover.