Jacob ben Reuben (יעקב בן ראובן) was a Karaite scholar and Bible exegete of the eleventh century.
But Jacob also drew upon later Karaite authors, the last of whom is Jeshua ben Judah, who, so far as is known, flourished about 1054 (see Harkavy, Hadashim gam Yeshanim, vii.
Among Rabbinitic authors Jacob quotes Abu al-Walid ibn Janah; but his quotations have apparently been intentionally suppressed by Firkovich in his edition (see Abraham Harkavy, Altjüdische Denkmäler aus der Krim, p. 211, note 1), though they are found in the manuscripts, and one of them has been given in the edition (on Jer.
If Jacob read Abu al-Walid not in the Arabic original but in the Hebrew translation, he must have compiled his book in the second half of the twelfth century.
Abraham Firkovich believes Jacob to have lived at Kerch, in southern Russia, said to have been called in Hebrew; and he asserts that the quoted several times in the commentary to the Pentateuch is identical with Abraham ben Simhah of Kerch (c. 986), a personage invented by him.
Jacob was probably a native of Constantinople, as his commentary contains Greek language glosses; and he appears to have been influenced by Byzantine authors.
The Sefer ha-'Osher (as of the beginning of the twentieth century) was found in manuscript at St. Petersburg, Paris, and Leyden.