His formal schooling ended at fourteen when, as "a chubby fair-complexioned boy, dressed in a blue corduroy suit".
Osmyn began his apprenticeship at the printer's trade in the shop of Samuel Armstrong on Cornhill, Boston. Seven years of apprenticeship in the "mystery" of this trade were so many years of real education for Brewster; each day's education began at six; there was an hour from seven to eight for breakfast and an hour for dinner; in summer the day ended at dark, but after September 20 it continued after eight by candlelight until ten.
At twenty-one, Osmyn Brewster and a fellow-apprentice, Uriel Crocker, were taken by their employer Samuel Armstrong on Cornhill, Boston into partnership. The first stereotyped edition (1824) of Scott's Family Bible bears the imprint of S. T. Armstrong, and Crocker and Brewster. Brewster's work was in the bookshop with Armstrong and he carried on that part of the business after the latter's retirement from the firm in 1825 and the consequent change in the firm name to Crocker & Brewster. Books of a religious nature were the chief output so that the bookshop got the reputation of being "the great mart of religious literature for the Orthodox churches".
Worcester's edition of Watts' Psalms and Hymns is listed in the firm's catalogue of 1830; so great was the demand for this hymnal that at least four editions followed. Among the school texts published by the firm was Ethan Allen Andrews' First Lessons in Latin, and a very complete set of Latin texts by the same author.
A very different kind of publication was the series of juvenile books by J. S. C. Abbott, one of the earliest titles being The Child at Home; or, The Principles of Filial Duty. The firm's imprint will be found on no book of doubtful character or of questionable morality. His was the only Boston publishing house which survived the financial panic of 1837.
In 1848 he was elected to represent Boston in the Massachusetts House of Representatives, and on four other occasions his fellow citizens honored him. For one term (1853) he was a state senator, and he was a delegate from Boston to the state constitutional convention of 1853. He also served for three years (1856 - 58) on the Boston board of aldermen. He became the first president of the Franklin Savings Bank in 1881 and continued to serve in that capacity until 1887.
One of the organizations which had his ardent support was the Massachusetts Charitable Mechanic Association, of which he was the treasurer for over twenty-five years. The Annals of that organization in 1853 was "from the press of Messrs. Crocker and Brewster" and "altogether a beautiful specimen of a book".
Brewster and his business partner Crocker that they sold out to Houghton & Company in 1876.
Brewster's home in the latter years of his life was at 32 Hancock St. , Boston.
At fourteen years old Brewster was described, as "a chubby fair-complexioned boy, dressed in a blue corduroy suit".
Osmyn Brewster married Mary Jones at Boston, January 15, 1824; she died, January 27, 1872. They had two sons and seven daughters.