- Oct 1 Cyclone strikes Calcutta: 70,000 killed
- Oct 1 John Summerfield Staples is paid $500 as a substitute for US President Abraham Lincoln
- Oct 2 Battle of Saltville, Virginia
- Oct 4 National black convention meets in Syracuse, New York
- Oct 4 New Orleans Tribune, 1st black daily newspaper, forms
- Oct 5 Battle of Allatoona, 1/3 of Union troops die repulsing South
- Oct 5 Most of Calcutta destroyed by cyclone, approx 60,000 die
- Oct 7 -Oct 13th) Battle of Darbytown Road, Virginia
- Oct 7 Naval Engagement at Bahia Harbor, Brazil - CSS Florida vs USS Wachusett
Victory in Battle
Oct 9 Battle of Tom's Brook; Confederate cavalry that harassed Sheridan's campaign is beaten by General George A. Custer & Merrit's cavalry divisions
Historic Event
Oct 12 War of the Triple Alliance begins with Brazilian troops under the command of Gen. João Propício Mena Barreto invading Uruguay
- Oct 13 Battle at Darbytown Road, Virginia (337 casualties)
- Oct 13 Maryland voters adopt new constitution, including abolition of slavery
- Oct 15 Confederate troops occupy Glasgow, Missouri
- Oct 19 Approx 25 Confederates make surprise attack on St Albans, Vermont
- Oct 19 Battle of Cedar Creek, Virginia, Union beats back Confederate attackers
- Oct 19 US Brigadier General Emory Upton (25) promoted to Major General
- Oct 21 Battle of Westport, MO (Kansas City)
- Oct 23 Battle of Westport, Missouri: Union General Samuel R Curtis defeats Confederate General Stirling Price
- Oct 25 Battle of Marais Des Cygnes River, Kansas (Mine Creek)
- Oct 26 Union troops ambush & kill 'Bloody' Bill Anderson near Albany, Missouri
- Oct 27 Battle of Boydton Plank Road, VA (Burgess' Mill, Southside Railroad)
- Oct 27 Second Battle of Fair Oaks, Virginia near Richmond
- Oct 27 Confederate ship Albemarle torpedoed and sunk by a spar torpedo mounted on a steam launch
- Oct 28 Second Battle of Newtonia (American Civil War), Newton County, Missouri
- Oct 28 Second Battle at Fair Oaks, Virginia, ends with 1554 casualties
- Oct 28 Battle of Wauhatchie, Tennessee
- Oct 29 Greek parliament accepts new Constitution
- Oct 30 Helena, Montana's capital, founded
- Oct 30 Treaty of Vienna signed to end the Second Schleswig War - Denmark loses control of Schleswig to Prussia, and of Holstein to Austria
- Oct 31 Nevada admitted as 36th state of the Union