Ohio Rev. Code Ann. § 327.04

The board of trustees of the sinking fund shall invest all moneys subject to its control in bonds of the United States, of this state, or of a municipal corporation, school district, township, or county of the state, and shall hold in reserve in cash only such sums as may be needed for effecting the purposes of sections 327.01 to 327.08, inclusive, of the Revised Code.