He was the author of a commentary on the Republic of Plato, which is lost; as well as of the Strategikos (Ancient Greek: Στρατηγικός) - a short but comprehensive work on the duties of a general, which was dedicated to Quintus Veranius.

The only details known about Onasander's life are from his own work, the Quintus Veranius to whom he dedicated the Strategikos to was the consul of A.D. 49, who died while in command in Britain ten years later, so the terminus ante quem for the composition of the treatise is 59.

He regards everything from the point of view primarily of the commanding officer, to the question of selecting whom he devotes a long and valuable passage, and he lays uncommon stress upon ethical and religious considerations... there is nothing very philosophic nor technically military in the treatise, which is intended to give merely the broad principles of generalship (στρατηγικαὶ ὑφηγήσεις, prooem.

But the essence of the first idea is as old as Nestor's advice in the Iliad (Β 362 f.); it was practised among the Eleans, Italic Greeks, Cretans, and Boeotians, being characteristic of the Sacred Band of Thebes, and something similar may not have been unknown at one time in Sparta, hence it can hardly have escaped the attention of military writers.

On the contrary, one would rather be inclined to wonder that, in an ethical study of warfare like the present, a commentator upon Plato's Republic should have failed to show at any point some trace of the not infrequent references to war and its basic cause, the character of the good soldier, the need of constant military exercise, the style of life of the soldier, the professional aspect of successful military preparation, mathematics as a necessary element in an officer's education, proposals looking toward the elimination of certain of the more cruel aspects of warfare, at least between civilized states, and similar topics discussed in that great work.