The collection is arranged in six series: Incoming Correspondence (1887-1937), Miscellaneous Correspondence (1887-1965), Financial Records (1895), Writings (undated), Miscellany (1863-1985), and Clippings (1882-1976, undated). The Writings are divided into four sections: poetry, poetry used as lyrics, prose, and "Reminiscences and Poems of Early Montana and the Cattle Range." This last section is composed of poetry and prose arranged by O'Malley in book form, which apparently was intended for publication. Included in this "book" are three pieces not written by O'Malley. Included in the Clippings series are three scrapbooks compiled by O'Malley, which include articles about him, poems and prose written by him, and articles and poems of interest to him, mostly about people he knew or occurrences in the frontier west.