Thats a good question Apparently some early authors maybe even John Adams thought that Otis wrote the letter but James Hosmer one of the first professional historians to write on the topic had this to say in his  biography of Samuel Adams  The authorship of these documents has been claimed for Otis the assertion being made that Adams was concerned with them only as his assistant The claim is however quite untenable In style and contents they reflect Adams while they are in many points inconsistent with the manner and opinions of Otis Aside from the strong internal evidence the most satisfactory external proofs have been produced  As far as I know subsequent historians have followed Hosmers lead on this point