The Correspondence series (1889-1949; 99.5 linear feet) is by far the largest series in the Osborn papers. As a public figure, Osborn corresponded widely and frequently with other notables, business associates, and a legion of acquaintances and admirers who had known him as newspaper publisher, governor, businessman, mining and land speculator, and author. The total of Osborn letters in the collection probably numbers close to 200, 000 items. Osborn corresponded with nearly every major Republican political leader in Michigan, as well as with many minor politicos within the party, prominent Democratic Party leaders, and other newspapermen. Osborn's business and political interests and his research projects are obviously principal topics within the letters. Included are correspondence from every president from McKinley to Truman. Of special note is correspondence with many prominent Progressives including Theodore Roosevelt concerning the campaign of 1912, and material on the elections of 1904, 1908, 1910, 1912, 1916, 1918, 1920, and 1930. Osborn's own campaigns for governor in 1910 and his candidacy for the U.S. Senate in 1930 are also well documented.
The chronological arrangement that Osborn maintained for his files, especially in a collection of this size poses some difficulties for the researcher. As his correspondence is not arranged by topic, the researcher must display some diligence in uncovering the great wealth of information that Osborn's communications contain. Essential is a sense of the chronology of Osborn's life. With that, the researcher will be rewarded with correspondence with significant individuals discussing the issues of the day: progressivism, conservation, Upper Peninsula development, state government, and national politics. There is, for example, correspondence showing progressive concern over business-government relations (cf. correspondence with Frederick D. Underwood, president of the B & O, and the Pennsylvania). The correspondence also includes much discussion of foreign relations: British colonial policies, the Boer War, British-German, British-United States, Canadian-United States relations, the Russo-Japanese War, China, and Chile and the Philippines. Other topics include contemporary political, social and intellectual issues: imperialism, nationalism, Anglo-Saxon racism, regulation of utilities and railroads, monetary and international trade policies, conservation, radicalism and socialism, German immigration, Anti-Semitism, Anti-Mormonism, Anti-Catholicism, prison reform, urban redevelopment and beautification, prohibition, and woman's suffrage. To aid the researcher, the library has prepared a separate index to the correspondents and a selective index to topics. This card inventory is located in the Reading Room of the library. Topics indexed in this file include Anglo-Saxon racism (1898-1899), progressivism and anti-progressivism (1908-1919), prohibition (1914-1926), forest conservation (1907-1926), labor and labor unions (1910-1921), and woman suffrage (1901-1917).
The Correspondence for the period of 1889 to 1908 is arranged alphabetically within a specific period of time, usually a few months. Apparently, Osborn maintained his correspondence in letterboxes. When one filled up, he started another. The correspondence for 1909 to 1949 is arranged only by chronology. These files might have been arranged alphabetically like the earlier files and then rearranged at a later date into a solely chronological sequence.
The Correspondence series also includes card index to correspondence (1 small box).