Just can share my experience booking an OEBB 1st class ticket (MUC-VIE) on Deutsche Bahn. You can book the train but not the reservation. One reason for this is that OEBB opens up seats for reservation only a few weeks before while Deutsche Bahn let's you book much earlier. Second it seems Deutsche Bahn and OEBB still struggling on the IT interface to process seat reservations. So what I did was to book the ticket on Deutsche Bahn and to book the reservation separately on OEBB when it was available. Problem with this is that normally on Deutsche Bahn the seat reservation for 1st class is included int he ticket but now I had to pay for it. Still better than not having a seat (it was a holiday and 1st class was completely full).