Ohio Rev. Code § 703.011
In addition to the method of determining classifications of municipal corporations set out in section 703.01 of the Revised Code, villages, which at the most recent general election had more than five thousand resident electors registered with the board of elections of the county in which the village is situated or, when more than five thousand resident electors have voted at the most recent general election, in such village, shall become cities. In such case, the board of elections of the said county shall immediately certify to the secretary of state the number of resident electors registered or voting within said village, and the secretary of state, upon receiving said certification from the board of elections, certifying that there are five thousand or more resident electors registered or voting within said village, shall issue a proclamation to that effect. A copy of the proclamation shall forthwith be sent to the mayor of the village, which copy shall forthwith be transmitted to the legislative authority of the village, read therein, and made a part of the records thereof. Thirty days after the issuance of the proclamation the village shall be a city.
R.C. §703.011