With submitting your PhD thesis, you need to apply for admission to the oral examination. Both has to be done online via a single Lucom form. Required documents and further information can be found under PhD Thesis or the Checklist - Countdown to doctoral degree.
The oral exam known as the thesis defense (or disputation) usually is scheduled up to six weeks after submission. At least four members of the examination board (including the two referees) have to attend the oral examination. The defense lasts 60-90 minutes and is open to University members. The first 30 minutes are reserved for a presentation where the doctoral candidate is placing his/her dissertation in a larger scientific context. This is followed by 30-60 minutes of questions, which are initially only permitted by members of the examination board. However, this board or its chairperson ensures that afterwards members of the audience are able to ask questions within reasonable limits as well. Finally, the committee is secluding to decide on an overall grade for the doctoral examination.
Oral Examination
A list of upcoming PhD defences can be found here.