Civil War Union Army Officer. Born in Country Londonderry, Ireland, he was a tavern owner in Philadelphia, Pennsylvania and a member of the 2nd Pennsylvania Militia regiment before the Civil War. When that conflict started, he help recruit the 24th Pennsylvania (Three-Month) Volunteer Infantry, a unit that had the 2nd Pennsylvania Militia as its nucleus. Commissioned Major, Field and Staff, on May 1, 1861, he was with his regiment has it served first in Maryland and then Virginia before their enlistment expired in July 1861. In August 1861 he joined with many of the men from the 24th Pennsylvania in re-enlisting to continue the war effort, and they formed the basis of what became the 69th Pennsylvania Volunteer Infantry. Commissioned Lieutenant Colonel, Field and Staff on August 19, 1861, his new regiment was composed largely of Irish immigrants like himself, and they would emblazon the Irish harp on their flag. The unit eventually was joined with the 71st, 72nd and 106th Pennsylvania Volunteer Infantry regiments to form the famous “Philadelphia Brigade”. Dennis O’ Kane would serve as second-in-command through 1862, participating in the Peninsular Campaign of May and June, the 2nd Bull Run campaign in August, and the September 1862 Battle of Antietam, where his brigade was caught in the West Woods area and took heavy losses. In November 1862 the 69th Pennsylvania’s commander, Colonel Joshua T. Owen, was promoted to Brigadier General, US Volunteers, Dennis O’Kane was advanced to Colonel on December 1, 1862 to fill the vacancy. At the December 14, 1862 Battle of Fredericksburg, Virginia he would lead his men in the third of four waves of futile Union charges on strong Confederate positions at Marye’s Heights south of the town, and he would see his regiment sustain fifty one casualties. In May 1863 during the Chancellorsville Campaign, his brigade would be held in reserve and saw limited action. During the July 1863 Battle of Gettysburg, Pennsylvania, he would find the 69th Pennsylvania would be positioned along a rock fencing in the middle of the Union lines that would become famous as “The Angle”. That position became the epicenter of “Pickett’s Charge” on the Third Day of the battle (July 3, 1863), as the remnants of the Confederate forces, having been much devastated from Union artillery fire, crashed over the rock walls and engaged the Philadelphia Brigade in brutal hand-to-hand fighting. Colonel O'Kane was shot in the head at the wall and died the next day; his regiment covering taking again fearful casualties but succeeded in helping to repulse the rebels and defeat the charge. The monument for the 69th Pennsylvania Infantry in Gettysburg Military Park stands on the spot where Colonel O’Kane was mortally wounded.