The Postdoc Association committee organises different career-oriented talks and workshops for postdocs (grant writing, transition from academia to industry, peer review, leadership for postdocs...), which sometimes complement those organised by the Graduate Students Association. Get in touch with the Postdoc Association committee to know more about these or, if you wish to get involved.
Furthermore, the Careers Service at the University offers one-to-one appointments to give feedback on your CV, or provide training for interviews. In combination with the regular careers fairs that University provide, there will be no shortage of advice and guidance for Postdocs looking at their next step on the career ladder.
More generally, the Oxford Learning Institute (OLI) provides a range of courses that will endow Postdocs with a wide range of new skills, important for their current career but also skills (ranging from statistics to leadership and time management) that will be transferable to other professions.