He attended the University of Rochester, where he was a member of the Iota chapter of St. Anthony Hall aka the Fraternity of Delta Psi.

During the American Civil War, Otis was appointed captain in the 140th New York Volunteer Infantry Regiment formed in Rochester, NY in September 1862.

During the Siege of Petersburg, he assumed command of the 1st Brigade, 2nd Division in V Corps leading it into action at the Battle of Peebles' Farm.

Otis continued serving in the army during the Indian Wars as part of the 22nd U.S. including campaigning in Montana in the aftermath of the Battle of the Little Bighorn.

Otis assumed command of the Eighth Army Corps, replacing Merritt, who had become the military governor of the Philippines.

"[3][4] In May 1899, the American Anti-Imperialist League published a pamphlet of letters allegedly written by U.S. soldiers in the Philippines that documented numerous atrocities committed by troops, leading to a wide public backlash against Otis and the war.

[5] In response, Otis denied to the press that any abuses had been committed, and tracked down the authors of the letters, threatening them with a court-martial if they did not immediately write a retraction.

[7]: 27–29  On the other hand, Rudolph Rau writes of Otis' work in the Philippines that "He delegated no authority, was pompous and fuzzy, and inspired few".