There is no university called Oxford Trent University. There is the University of Oxford and Trent University (in Canada). Neither of these universities are known for offering online PhD programs in the same way some other universities do. While some universities may offer *some* components of PhD programs online, a full online PhD is less common, especially at prestigious universities.
To find out if either Oxford or Trent offers any online PhD components or programs, you should check their official websites.