One of the reasons why one will need to do an oral defence of their dissertation is that they have to convince their dissertation committee that they are conversant and an authority in the kind of research that they have done. You have to explain to the committee the manner in which you have carried out the research as well as the way it fits with the other related scholarly work and research done on the same field. In most institutions, one has to defend their thesis or their proposal.
The dissertation defence normally lasts for one to two hours. The dissertation defence could be termed t o be some form of examination; however, the presenter actually knows more about the topic than the examiners. For this reason, one needs to be comfortable in their presentation as they are truly the authority in that subject at that particular moment! During the actual presentation, you will need to stay calm and smile a little bit. This will show the participants that you are confident of the information that you are about to present. It is important for the candidate to listen carefully to all the questions that will be asked by the committee.
Academics may be lengthy in the manner in which they frame their questions. You have to ensure that you have understood all the questions before you can attempt to answer them. Now, if you do not understand the question that has been asked to you, you need to pause and get it right. You may not also be able to answer all the questions that the dissertation committee or the panel will ask you. You do not have to know all the answers to all the questions. Just acknowledge that you do not sure about answer and take a stab on it. This is better than trying to fake it. Finally, you must be well groomed and smart for your presentation. This is very important.