Oldcastle is an unincorporated community in the town of Tecumseh, Ontario, Canada. It is bounded to the north by Highway 3, to the east by the Chrysler Canada Greenway trail, to the south by County Road 8, and to the west by Howard Avenue. It is the starting point of the Chrysler Canada Greenway rail trail. Its western half consists primarily of industries and businesses, while the remaining area is farmland and residential.