Most of his later writings were occasional pieces published in periodicals, some of very limited circulation, which explains why his work has only recently come to enjoy the considered attention of literary scholars. O'Brien was also notorious for his prolific use and creation of pseudonyms for much of his writing, including short stories, essays, and letters to editors, and even perhaps novels, which has rendered the compilation of a complete bibliography of his writings an almost impossible task. Under pseudonyms, he regularly wrote to various newspapers, particularly "The Irish Times", waspish letters targeting various well known figures and writers; mischievously, some of the pseudonymous identities reflected composite caricatures of existing people, this would also fuel speculation as to whether his model (or models) for the character was in fact the author writing under a pseudonym, apparently leading to social controversy and angry arguments and accusations. He would allegedly write letters to the editor of "The Irish Times" complaining about his own articles published in that newspaper, for example in his regular "Cruiskeen Lawn" column, or irate, eccentric and even mildly deranged pseudonymous responses to his own pseudonymous letters, which gave rise to rampant speculation as to whether the author of a published letter existed or not, or who it might in fact be. There is also persistent speculation that he wrote some of a very long series of penny dreadful detective novels (and stories) featuring a protagonist called Sexton Blake under the pseudonym Stephen Blakesley, he may have been the early science fiction writer John Shamus O’Donnell, who published in "Amazing Stories" at least one science fiction story in 1932, while there is also speculation about author names such as John Hackett, Peter the Painter, Winnie Wedge, John James Doe and numerous others. Not surprisingly, much of O'Brien's pseudonymous activity has not been verified.