The closest airport is Düsseldorf (DUS) with a direct train line to Oberhausen main station (between 30 and 40 minutes travel time): Google Maps link

The Ryanair airport Weeze (NRN) is also fairly close. There is an express shuttle bus to Duisburg main station. It is usually a five minute train ride to Oberhausen main station: Google Maps link

The airport Cologne-Bonn (CGN) is also well connected to Oberhausen main station. Travel time between 60 and 90 minutes: Google Maps link