My ancestor petitioned for land in Fairfield County in 1799. He received a deed to his property in 1805. He was on the tax rolls beginning in 1806. His daughter's 1802 marriage was one of the first recorded events in county history. If Fairfield County was a part of some other county before that, I am not aware of it. As far as I know, Fairfield County was a part of the Northwest Territory prior to the land being opened up to petitioners in the late 1790s. The exact county lines may have changed somewhat over time, but I think Fairfield was an original county.

From the Fairfield County GenWeb site: Fairfield County, eighth county to be formed in the Northwest Territory, was created by the Governor and Council of the Northwest Territory, and was proclaimed by Governor Arthur St. Clair on December 9, 1800. Both county and seat of justice are older by three years than the State of Ohio. The land area of the new county included the present counties of Delaware, Knox, and Licking, with large portions of Franklin, Perry, Pickaway, and Hocking.