The nearest service to OSH by the majors is Appleton (ATW). It’s only 20 minutes or so north of Oshkosh. You can get into ATW via Minneapolis, Detroit and Chicago and others. There is bus service out of MKE north but no express service. Limo service is pricey. Rental cars are worth their weight in gold during AV. If you’re planning on attending book your reservations early. Welcome to Oshkosh!!!