The Ohio New Establishment Dynamics (O-NED) data set tracks the number of establishments that first started employing people between the second quarter of 1997 and the first quarter of 2008 and measures the employment and payroll data for these new establishments. O-NED enables researchers to measure the growth trends of cohorts of new establishments for up to 5 years after the cohort’s birth. These data are the first publicly available data that document the growth rates of new establishments at the substate level. The finest unit of geography O-NED measures is a county. This article describes how O-NED is constructed and defines the variables included in the data. It closes with two examples of how researchers can use the data.