(Henry), a learned German gentleman in the 17th century, was descended from the noble family of his name, who were earls of the county of Oldenburg, in the north part of Westphalia for many generations. He was born in the duchy of Bremen in the Lower Saxony; and during the long English parliament in king Charles I.'s time, was appointed consul for his countrymen, at London, after the usurpation of Cromwell: but being discharged of that employ, he was made tutor to the lord Henry O'Bryan, an Irish nobleman, whom he attended to the university of Oxford, where he was admitted to study in the Bodleian library in the beginning of the year 1656, when Cromwell was vice-chancellor. He was afterwards tutor to William lord Cavendish, and was acquainted with Milton the poet. During his residence at Oxford, he became also acquainted with the members of that society there, which gave birth to the royal society; and upon the foundation of this latter, he was elected fellow; and when the society found it necessary to have two secretaries, he was chosen assistant-secretary to Dr Wilkins. He applied himself with extraordinary diligence to the business of his office, and began the publication of the Philosophical Transactions with No. 1. in 1664. In order to discharge this task with greater credit to himself and the society, he held a correspondence with more than seventy learned persons, and others, upon a vast variety of subjects, in different parts of the world. This fatigue would have been insupportable, had not he, as he told Dr Lister, managed it so as to make one letter answer another, and that, to be always fresh; he never read a letter before he had pen, ink and paper, ready to answer it forthwith; so that the multitude of his letters cloyed him not, nor ever lay upon his hands. Among others, he was a constant correspondent of Mr Robert Boyle, with whom he had a very intimate friendship; and he translated several of that ingenious gentleman's works into Latin. Mr Oldenburg continued to publish the Transactions, as before, to No xxxvi. June 25, 1677. After which the publication was discontinued till the January following, when it was again resumed by his successor in the secretary's office, Mr Nehemiah Grew, who carried it on till the end of February 1678. Our author dying at his house at Charleton, near Greenwich in Kent, in the month of August that year, was interred there.