On ÖBB long distance trains, you can reserve any seat you wish. Depending on the type of wagon, you have different options: from seats to couchette berths, from open cars to closed compartments.
Warning: Tickets for some trains - such as the ÖBB night trains - include a valid seat reservation.
Information on which trains allow or even require a seat reservation can be found in the ÖBB rail schedule.