The Union of Orthodox Jewish Congregations of America was founded as a lay synagogue federation in 1898 by Rabbi Henry Pereira Mendes.

[3] Cracks between the OU and JTS first formed in 1913, when Solomon Schechter decided all alums of the institution would be allowed to apply for managerial positions in the newly created United Synagogue of America (then a non-denominational communal organization), though his close ally Rabbi Frederick de Sola Mendes advocated that only strictly pious ones should be so approved.

Only then did Mendes begin to distinguish between "Conservative" and "Modern Orthodox" Judaism in his diary, though he could not articulate the difference.

[5] During the early decades of its existence, the Orthodox Union was closely associated with and supported the development of Yeshiva University into a significant Jewish educational institution producing English-speaking, university-trained American rabbis for the pulpits of OU synagogues.

Because of this, after much negotiation, the Orthodox Union agreed to drop the word "kosher" from their initial design in favor of the less Jewish-sounding "OU" symbol.

[9] The OU's kashrut program was heavily influenced by Abraham Goldstein, a chemist who used his knowledge of food science to determine the kosher status of various products.

The wide acceptance of OU kashrut supervision rested largely upon the outstanding reputation of its rabbinic administrator, Rabbi Alexander S. Rosenberg.

Bernstein became the founding editor, in 1951, of Jewish Life, the OU's popular publication for Orthodox laymen.

During the postwar years, there was considerable overlap in the lay leadership of the Orthodox Union and Yeshiva University.

Its leadership ranks were augmented by a talented group of lay leaders, including Joseph Karasick, Harold M. Jacobs, and Julius Berman, who would guide the OU's growth over the next several decades.

By inspiring thousands of public-school educated high school youth across North America to become more observant, NCSY played a major role in launching the baal teshuva movement, a widespread spiritual re-awakening among Jewish youth which followed the 1967 Six-Day War.

By the 1990s and early 21st century, the OU's general philosophy and observance levels have shifted towards stricter interpretations and halachic practices.

This change has not necessarily affected individual member congregations but has impacted many Orthodox Jewish communities across America.

Given Sessions' policies, particularly those concerning immigrants and asylum seekers, the OU came under criticism for hosting him and presenting him with the plaque.

It has now expanded its reach to include many already religious, mostly Modern Orthodox children attending Jewish day schools.

Many OU synagogues support the concepts of Religious Zionism, which teaches that the existence of the State of Israel is a step towards the arrival of the Messiah and the eventual return of all Jews around the world to live in the ancient national Jewish homeland.

Until the 1980s,[19] the most popular English translation of the prayer book used in OU synagogues had been Ha-Siddur Ha-Shalem edited by Philip Birnbaum.

[21] The OU has been accused of ignoring multiple reports of child abuse when appointing Rabbi Baruch Lanner as Director of Regions of its National Conference of Synagogue Youth movement.

In 2006, the OU's response was the subject of a video narrated by Jonathan Safran Foer, Irving Greenberg, and David Wolpe.