Mary Jane Thurston State Park
From TrailGuide
Mary Jane Thurston State Park is in northwest Ohio, southwest of Toledo near the town of McClure. Though it doesn't mention it on the park web page there are trails open to mtb:
http://www.dnr.state.oh.us/parks/parks/mjthurstn/tabid/768/Default.aspx
I've corresponded with the park and they've told me that the trails at the North Turkeyfoot area are open to mountain bikes (and horses). Here is the link to the park map:
http://www.dnr.state.oh.us/Portals/2/parkmaps/maryjhurstonparkmap.pdf