Katherine Vissering "Kitty" Oppenheimer (née Puening; August 8, 1910 – October 27, 1972) was a German American biologist, botanist, and a member of the Communist Party of America until leaving in the 1930s.

Her husbands were Frank Ramseyer, Joe Dallet, Richard Stewart Harrison, and physicist J. Robert Oppenheimer, the director of the Manhattan Project's Los Alamos Laboratory during World War II.

Her mother was, in fact, a cousin of Wilhelm Keitel, who later became a field marshal in the German Army during World War II, and was hanged in 1946.

Her father, a metallurgical engineer, had invented a new kind of blast furnace, and had gained employment with a steel company in Pittsburgh, and the family settled in the suburb of Aspinwall, Pennsylvania.

It is doubtful that she took any classes, but she did meet Frank Ramseyer, an American studying music in Paris under Nadia Boulanger, before sailing for home on May 19.

He had been involved in the International Unemployment Day protest in Chicago on March 6, 1930, that was brutally repressed by the authorities, and worked as a union organizer with the steel workers in Youngstown, Ohio.

"[12] The last letter from Dallet said that he was heading to Spain on the RMS Queen Mary to join the International Brigades fighting in the Spanish Civil War.

After a few days there, she returned to London, and they headed south, crossing into Spain[13] where he joined the Mackenzie-Papineau Battalion, a unit composed of American and Canadian volunteers.

Her trip to Spain was delayed by hospitalization for an operation on August 26, 1937, for what was initially thought to be appendicitis, but which was determined to be ovarian cysts, which were removed by the German doctors.

It was at a garden party thrown by Lauritsen and his wife Sigrid in August 1939 that she met Robert Oppenheimer, a physicist who taught at Caltech for part of each year[20] and the remaining time at University of California, Berkeley.

[24] The Serbers loved to ride through the pine and aspen forests and floral meadows of the Sangre de Cristo Mountains, camping with minimal food and equipment.

The following day Page rode to Perro Caliente on her bay horse to return Kitty's nightgown, which had been left under Robert's pillow.

Soon after, Robert shared a podium with Nelson to raise money for refugees from the Spanish Civil War, and he informed him that he was engaged to Kitty.

To obtain a divorce, Kitty moved to Reno, Nevada, where she stayed for six weeks to meet the state's residency requirements.

The divorce was finalized on November 1, 1940, and Kitty married Oppenheimer the following day in a civil ceremony in Virginia City, Nevada, with the court janitor and clerk as witnesses.

The holiday was marred when Oppenheimer was trampled by a horse, and Kitty injured her leg when she hit a car in front of her while driving their Cadillac convertible.

[28][29] The United States entered World War II in December 1941, and Oppenheimer began recruiting staff for the Manhattan Project.

[32] She put her biologist training to use working for the director of the Health Group at Los Alamos, Louis Hempelmann, conducting blood tests to assess the danger of radiation.

[36][37] During their time at Los Alamos and continued throughout their 26 year long marriage, the Oppenheimer couple strongly relied on each other to be their biggest confidants.

[38] The revoking of Robert Oppenheimer's security clearance not only revolved around his association with the communist party, especially through Kitty, but also issues with his bold opinions that many scientists and officials did not agree with.

[43] In 1947, Oppenheimer accepted an offer from Lewis Strauss to take up the directorship of the Institute for Advanced Study in Princeton, New Jersey.

[44] The job came with rent-free accommodation in the director's house, a ten-bedroom 17th-century manor with a cook and groundskeeper, surrounded by 265 acres (107 ha) of woodlands.

Robert had a greenhouse built for Kitty, where she raised orchids; for her birthdays Oppenheimer had rare species flown in from Hawaii.

[51] Kitty had his remains cremated and his ashes were placed in an urn, which she took to St. John and dropped into the sea off the coast, within sight of the beach house.

"[34] Kitty never remarried, but she spent the rest of her life with Robert Serber, an American physicist who also worked on the Manhattan Project.