He was the son of Adam Friedrich Adamson Graf von Moltke (1816–1885) and Fanny Charlotte Anna Luise Gräfin zu Rantzau (1824–1866).

He was educated in the gymnasium of Lübeck together with his older brother Karl, after his father's change of residence to that city due to the Prussian annexation of the province he administered.

[8] Due to his brave conduct in battle, Otto was promoted to standard-bearer and then, in September, to second lieutenant, also receiving the Iron Cross Second Class.

[1][8] Both, informed of the good working conditions and the prosperity of Chile through the German consul in Hamburg, Mr. Schutte, decided to immigrate to that country.

[1] Soon he and his brother found a modest job in a sugar refinery of Viña del Mar, which belonged to the German businessman and resident Julio Bernstein.

[9] Otto lived for three years in Santiago, and in his free time he cultivated his social relationships with the affability and simplicity that endeared him to those who knew him.

[9] When hostilities began against Peru and Bolivia in early 1879, Otto obtained a leave of absence from his work to volunteer in the Chilean Army in the face of impending war.

On 11 December, while in Pisco, he wrote a letter to his brother Karl about his good luck in the war and his impressions on the upcoming campaign for the Peruvian capital.

[12] Subsequently, on 26 December, after the Chilean landing in Curayaco, south of Lima, Otto, while on board the auxiliary cruiser Angamos, wrote his last letter to his brother Karl, in which he said the following:[12] My dear brother Karl, at this moment we have just anchored in the bay (of Curayaco); the rest of the army is already in Lurín, a point that, according to estimates, will be attacked by the Peruvians.

In that place the Chilean forces were repelled with several casualties by the intense fire, having to retreat and abandon some trenches that they had managed to occupy in the face of the Peruvian counterattack.

[13] It was during this part of the battle that he was mortally wounded by a gunshot to the chest, and then his body was cruelly torn to pieces on the ground by Peruvian soldiers with rifle butts and knives.