Can a Ohio landlord introduce a new fee mid-tenancy?

Generally no, on a fixed-term lease. The lease locks the rent and any specified fees for the term; introducing a new fee (parking, pet, trash, key replacement) without tenant agreement is a unilateral lease modification, not enforceable. On a month-to-month tenancy in Ohio, the landlord can add a new fee with the same advance notice required for a rent increase (Ohio has no statewide advance notice specific to rent increases. ORC 5321.17 sets 30 days as the month-to-month termination default, treated as the default for rent changes. No state rent cap.). Once the notice period passes, the tenant either accepts the new fee by continuing to pay or terminates the tenancy. Some fees are statutorily controlled regardless of the lease: late fees (no statewide cap), application fees, and fees that exceed the actual cost of the service. A "new" fee that effectively re-categorizes a previously-bundled cost is treated as a rent increase.

Source: ORC 5321.17 (30-day month-to-month termination default)