As a researcher a lot of your 'professional development' will involve publishing papers, attending conferences, and acquiring the skills you need to do your research well.
But it is also important to think about other areas beyond your research itself. Do you need to gain teaching experience? Could you run a conference or symposium to develop your project and budget management skills? Could you help write a grant application? Might attending a workshop on time management help you do your job better?
Being proactive about these other aspects of being a researcher will make your CV stand out when you apply for your next step, whatever that is.
There is a wealth of support at Oxford for you to develop in whatever areas you want to. It can sometimes be difficult to find as there are so many different providers - these pages pull together most of the opportunities available (though you'll want to ask your department what they offer, too).
If you're not sure what sort of development would be useful to you, have a look at the page on personal development review.
If you're looking for a course to develop your research or transferable skills or to help improve your effectiveness at work, the training and workshops page will point you towards the main providers.
Do you have any suggestions for training or resources that you would find helpful? Please let us know.