William O'Hara (d. July 21, 1821) was a Missouri land speculator who in 1819 purchased 1,120 acres worth of New Madrid federal certificates granted to survivors of the devastating New Madrid earthquakes of 1811-1812. Some of these certificates were for lands at the present site of Little Rock, Arkansas.
O'Hara became embroiled in conflict with rival speculator William Russell who held an overlapping claim derived from Little Rock first settler William Lewis. The conflict ignited the so-called New Madrid Land War in April 1821. O'Hara had accidentally built his proposed capitol on land indisputably owned by Russell. The town was moved in the middle of the night about three blocks west, from what would become Scott Street to the location of present-day Center Street: "[A]bout one hundred men, painted, masked, and disguised in almost every conceivable manner engaged in removing the town. These men with ropes and chains, would march off a frame house on wheels and logs, place it about three or four hundred yards from its former site and then return and move off another in the same manner." O'Hara called this new town Arkopolis, a name that never caught on.
O'Hara sold his certificates and exited the controversy that same month, and died three months later. He was replaced on the New Madrid side by Chester Ashley. Ashley lost the case in June 1821 in the Superior Court of the Territory of Missouri. The dispute continued to simmer until both the Russell and Ashley sides lost their lands under the Preemption Law of 1830.