Pro tip for this year: MOST airports are a total mess currently in the whole of Europe, you will probably not arrive at the time listed on your plane ticket. You can order your train tickets online now (https://www.dsb.dk/en/) but hold off on the seat reservation and get it in the DSB app or the ticket machine when you land (reserve a seat from Copenhagen Central (København H) to Odense St. No seat reservation needed from the airport to copenhagen central, it's only a very short ride.